Transport within Vienna

Here are all the details about the "Hop on Hop off Tours", i.e. the different routes, the stops, timetables, etc.:
https://www.viennasightseeing.at/en/hop-on-hop-off/

I don't think that it is allowed to board/alight the busses between stops.

Hotels in Vienna always serve a breakfast, normally included in the room price. So you should not need to travel to get some food in the morning.

If you want the Vienna Pass to pay off, you must visit a lot of sights, which are all at stops of the hop on hop off busses. I doubt that you will have time then for going elsewhere using the travel card option.

Thanks another question is do I need to buy the Vienna Pass/Salzburg & Innsbruck card right away? Can it be bought a few days before travel. My concern is that it should not get sold out

You can do the math on your specific plans to work out whether you should get the Vienna Pass or not. It is impossible to calculate based on the general information you have in your post. So . . ..

Without knowing more details of where you are staying and your sightseeing plan in Vienna, I would recommend you just buy a strip card for the public transit system. You stamp it for each trip on the transit system. It doesn't expire. Then you can use it the first days you are in Vienna or the last days. Transit in Vienna is cheap.

Also, know that the distances are short--the center of Vienna is quite compact. If your hotel is anywhere near the center or the Ringstrasse, you can probably opt to walk to some destinations if you wish.

For the Hop on stuff, you can get a ticket from the staff who stand at the major stops. They have maps and info. I don't think they reserve slots on given runs of the buses. You just queue up. I doubt it would be an issue--those buses never look to be completely full (unless you are travelling with a big group).

To be honest, unless you are working with a budget cut really close to the bone, I'd suggest you not micromanage your visit and leave time for flexibility. A strip card would allow for that. If by chance you have an extra trip left when you leave, just pay it forward and give your card away to someone at the airport and make their day.

you just buy a strip card for the public transit system. You stamp it for each trip on the transit system.

The strip card holds 8 strips, each worth for one day for one person. So you punch one strip only once per day on your first ride, not on every ride. Each strip is valid for a calendar day, not for 24 hours.

wmt, I do not mean the Klimakarte. I mean just the single trip tickets that come in a strip of 4 (and they used to have 8 trip strips). You stamp those for a trip, not a day. The Klimakarte is overkill for he_var since it's 8 days.

In any case, a 24 hour pass is only 7,60. If you take 4 trips in a day, you do better than buying single tickets at 2,20.

If he_var is not travelling alone, the Klimakarte would be the best choice.

Thanks all. We are a group of 8 people, 6 adults and 2 children (12 and 13 years old ).We are staying near Naschmarkt(about 500 M).I reckon it is not very far from the city center.But is that walkable?

Everything is walkable, even in the night. Vienna is a very safe city.
500 m = 6 minutes walking [500 meters = 0.3 miles]
Whether it is feasible to walk or to use public transport depends where you stay exactly. The Naschmarkt is about 500 m long, with an extension to the west for Saturday's flea market. The city end of Naschmarkt is about 500 m away from the State Opera.

Most tourists make the mistake to consider the distance to the center only when selecting a place to stay in Vienna, instead of looking for a good connection to public transport to avoid unnecessary walking.

I wouldn't bother with the hop-on hop-off unless you really want whatever narration it offers. As others have said, Vienna is walkable, and there's also plenty of public transportation between the trams along the Ring and the U-bahn.

I went ahead and bought a multi-day travel pass last summer; I had purchased an Austrian SIM card and so decided to purchase and use the pass on my mobile phone. (You can also buy the pass when you have wifi, take a screen shot of it, and then you'll have it even if you don't have mobile access everywhere.)