Should I start at the farthest or nearest country

When possible, most of us like to design itineraries that put our sightseeing in our departure city at the very end of the trip. If you do not do that, your last full day will be spent traveling to that city rather than vacationing. In your case, since you could potentially be in a completely different part of Europe, it would be really risky not to leave Austria (or at least Vienna itself) until the end of the trip. You might plan to fly back to Vienna from Dubrovnik, Barcelona, etc. What if that flight was canceled? You could be at risk of missing your flight home, which would be very costly.

What a lot of us try to do (not always successfully because of transportation schedules and uncertainty about when our flight will actually arrive and we'll get processed through Immigration) is move right on to one of our other planned destinations on our arrival day. Figuring out how to do that with the very least risk of missing onward transportation is tricky. When we can pull it off, we are happy that we have made productive use of the arrival day, when many of us are brain-dead from lack of sleep and jetlag. Since that day isn't much good for sightseeing anyway, we are glad to spend it covering some ground we'd need to cover at some point during the trip.

Edited to add: Unless there's a significant reason to do so (such as attending separate special events weeks apart), it's best not to have two stays in the same city on the same trip. It's inefficient to have to go through the process of finding the hotel, checking in, and getting settled in room twice for the same city.

If you want to stick to ground transportation on your arrival day, your practical options are heading to some other city in Austria that you plan to visit (probably fastest, and you're likely to have a lot more choices of departure time), or to Zagreb or Ljubljana. I think transportation to Zagreb might be a bit faster than to Ljubljana, but I haven't dug into the train and bus schedules.

I don't suppose Graz is on your list for Austria? It's quite a handsome city, and it's about halfway to Zagreb.

You could of course look for a cheap budget-airline flight to one of your more distant destinations, but it's hard to know how to time a separate flight like that, and you probably won't have as many departures to choose from if you have to fly.

Have you checked with your airline to see how much it would cost you to alter your airline ticket so you are not flying back from Vienna? The extra travel to get back to Vienna from Spain, Portugal or the UK is going to cost something. It's not just the cost of the ticket itself; if you're not traveling very light, you'll have luggage fees, and you'll have transportation to/from the two airports involved. Plus it will cost you a big chunk of one of your vacation days.

If your trip includes July or August and you plan to go to Madrid, Seville, Cordoba, Toledo or Granada, I'd try for an itinerary that has Spain at the coolest time on the calendar.

You may be able to change to an "open jaw" itinerary without paying a fee to the airline. Many airlines have stopped charging change fees because of changing travel rules for various countries and changes in travelers' health status. Have you looked into this?

If you're locked into a round trip to and from Vienna, then I'd suggest you fly directly from Vienna to Portugal (probably Lisbon) as soon as possible after landing in Vienna. Allow several hours to clear EU passport control and pick up any checked baggage, plus a cushion in case your flight to Vienna is delayed. From Portugal, make your way back toward Vienna as you choose. If you'll be visiting the UK, you might be taking some additional flights, since your plans don't include France.

The main reason I suggest Portugal first is to minimize the risk of missing your flight home at the end. The closer you are to Vienna (preferably in Vienna) just before that flight, the less risk. Otherwise, which is "better" is really up to you.

Edit -- Acraven said it better than I did, she has it exactly right, as usual.

Everyone has a different travel style. We often do a “loop back” to keep direct flights and especially if fares are lower.

In your case, I would head to Portugal and work your way back. We have done the “fly back to starting town to fly home” and it’s not a big deal; but next time I would leave that city for last definitely. It wasn’t memorable spending our last night in Paris eating airport food and staying at airport hotel for convenience.

That said, not everyone is up for flying in and flying right back out. Another factor to consider is jet lag. If it was me I’d book 1-2 nights and just relax there at arrival city walking around, enjoying parks and nature vs big sights or hard plans before hopping back on a plane- but I also know that I’d be maxed out after 14+ hours traveling and not want more airplane hassle but rather a walk outside and a good meal.

Lots of possibilities. If you stick with Vienna as your start and end point, you could start in Austria, then fly to Portugal, then work your way back to Vienna at the end.

If you wanted to split up the start and end locations, also known as doing an “open jaw” trip, which city gives you the best flight times? Not leaving for home too early in the day, meaning you have to get up extra early and arrive at the airport really early, if that’s any concern? And what flight times get you home in the shortest time, or at an acceptable time of day? Maybe the convenience of the flight schedule could help determine where to begin and end your trip.

Simple answer: farthest.

I often do this scenario of round trip from one city. I will split the time I want to spend in the arrival city in half, so in your case 4 days at the front end and 4 days at the back end. I love that I feel like I know the city I return to at the end. I will often see different sites than planned when I return thanks to my full experience on the ground. I hate making the arrival more miserable than necessary by adding more extended travel directly onto my long flight.

However, I am never as far away from my prime area as you will be, but not a big deal if you fly. Personally I would save Spain and Portugal for a later trip, but Spain seems to be a main interest for you so no big deal. It is your trip. That being said, senseless flights don’t help fight global warming and you will return to Europe.

What is best is what is best for you.

If you have the funds to do a 45+ day trip, I will assume you have the funds to pay the airline for an OpenJaws ticket (except now you will be subject to fuel surcharges and such). Not to mention, you are flying to Vienna, then traveling westerly, then traveling back easterly past Vienna, then back westerly to Vienna. If you do manage the UK you can travel a triangle which makes a little more sense.

That said, nothing wrong with a few days in City A, complete your travels, and then return to City A for a few days. First days there as a tourist, last days there for Rest and Repose.

A flight concept that is sometimes difficult to conceptualize ... Airliners don't always fly east to west. US to Europe generally flies over the Arctic Circle.

You mention that you are going in the summer. I would keep the expected temperatures in mind when planning my route.

It sounds like you have already purchased you tickets to and from Austria. However, you haven't planned your European travels yet.

First of all, I suggest keeping down flying within Europe as much as possible, since you loose a lot of time waiting in Airports. You can easily do Austria, Slovenia and Croatia without flying.

Also, perhaps you could do Spain and Portugal on another trip and instead visit Serbia, Greece and Istanbul in Turkey.

I would do the Austria/Slovenia/Croatia first and then move on to you next countries.

I vote for farthest first. When you go, you will have excitement as your motivator. When you return, you will want the shorter flying time to get back home.

I plotted your countries and it looks to me like this would be your best schedule. Your countries are so far apart you should travel by air in between your Vienna roundtrip.

Fly into Vienna and spend a few days to explore Vienna and rest up from your inbound flight.

Then fly from Vienna to Porto, Portugal. Schedule your travel through Portugal and through Spain to Barcelona and fly from Barcelona to Dubrovnik.

Schedule your trip from Dubrovnik through Croatia and Slovenia and then into Austria.

Finish up a day before your trip in Vienna.

The in between air fares are cheap and will save you a lot of time.

While probably not useful for your trip. By default I try to put the country i will have the easiest time communicating in last. This can be based on language or culture or whatever but by making the “easiest” country last it offsets the little annoyances that build up over time on a trip

I forgot to post, But I did my Itinerary already on all those countries that I mentioned on the topic. Also, the reason why I add UK, it's because I have a friend who live over there that I can visit.