First time travel to South East Asia

Sounds like a great trip - I'm sure you'll have fun.

What you're proposing is certainly possible, and is a fairly common itinerary. Personally I'm a less is more traveler, and wouldn't attempt it in a month, but it certainly can be done. Just don't underestimate the delays possible traveling by road.

There are cultural similarities (particularly between Thailand and Laos) but if you can visit all of these countries, do. I lived in Thailand and love it, and Vietnam is another favourite.

On your idea to visit Singapore or Tokyo or Seoul as well, can you fly into/out of Bangkok? That saves some time. If not, then Singapore is a good hub for the other countries you want to visit. There are lots of budget airlines here, so very easy to hop on a Scoot flight for example.

One concern I wluld flag with you considering you're thinking of hiking and outdoor activities is that it is probably the worst time of year to visit Northern Thailand, as it is burning season, and the air quality can be very poor. At the very least you need to be aware this may curtail your enjoyment of outdoor activities.

Vaccinations- beyond the standard tetanus shot, I don't have any other vaccinations. You may wish to consider Hepatitis A. Covering up to avoid mosquitos, which can spread dengue and malaria (depending on exactly where you are) is very important. Speak to your doctor and tell them exactly where you're planning to go, and what you plan to do, as rhis can impact whether you need vaccinations.

We are also planning on getting the required or recommended vaccines
for the trip but only want to get what is really necessary. Which
vaccines are typically recommended to get and are necessary? We only
want to get what is necessary.

Check the recommendations from the CDC, here is the page for Thailand but you should confirm for every country you plan to visit https://wwwnc.cdc.gov/travel/destinations/traveler/none/thailand

Before my trip to SE Asia besides my routine vaccines I also got TdAP, typhoid, and Hepatitis A. I strongly recommend visiting a clinic that specializes in travel, they are helpful making sure you get what you need that suits your personal health situation. Also some vaccines require a multi dose series before you have immunity, so now would be the time to start that process.

All this said, we are looking at some basic information to guide our
planning such as are these four countries and cultures significantly
enough different that we should endeavor to go to all four or is one
similar enough to a neighbor that we could realistically skip and not
miss very much?

Each country has a unique history and culture and it is not fair to assume they're all "similar" just because they are located in the same part of the world. Spend some time looking at what each country offers and decide to go (or not) based on the merits of each location based on your interests.

As for the best route, check out itineraries for tour companies for ideas. I personally opted for a tour to make the logistics much easier. Here is a recent thread that notes many different tour company options. https://community.ricksteves.com/travel-forum/beyond-europe/vietnam-tour-odyssey-unlimited-vs-road-scholar-vs

We’re in Japan now. We flew Cathay Pacific which just started nonstop service Seattle to Hong Kong, and then they have numerous routes from Hong Kong all over Southeast Asia. We were very impressed with the airlines service. They also offer free layovers in Hong Kong for up to 6 nights. We took advatage of this before we flew on to Osaka.

The other thing to highlight is that in addition to the (more expensive) full service carriers like Singapore Airlines, Cathay Pacific, Malaysia Airlines, Thai Airlines etc, there are numerous good budget carriers that fly around the region very cheaply and are excellent for short haul flights.

Take a look at Scoot, Air Asia, VietJet (great for Vietnam), Lion Air and Nok Air. To give you an idea, return Singapore-Ho Chi Minh City next month is USD120 on VietJet. The main budget hubs for the region are KL, Singapore and Bangkok. If you're flying from North Asia (HK, Seoul, Tokyo) it's a longer haul flight and therefore more expensive, but perhaps could work given you're coming from the US.

I’ve traveled independently in SE Asia and it certainly is not hard to do, but a few years ago I very much enjoyed Overseas Adventure Travel’s (OAT) tour called “Ancient Kingdoms”. It’s about three weeks, small group, and the itinerary through Thailand, Laos, Cambodia and south Vietnam combined major sites such as Angkor Wat with the kind of cultural immersion activities you’re looking for and that are more difficult to find on your own. I’ve taken a number of OAT trips and found the guides to be uniformly excellent and the fellow travelers to be congenial. Most are experienced older travelers such as yourselves, a mix of couples and singles. You could add some time on your own in Thailand or Vietnam before or after to fill out your month. My travel medical clinic didn’t recommend any more vaccinations as long as hepatitis A and other standard ones were up to date, and malaria prophylaxis was not needed for that itinerary and time of year (December dry season), but look to your own medical professional rather than an online forum for the most current definitive advice.

I would echo what Simon mentioned regarding budget airlines within Asia. Last month I flew Air Asia between Bangkok and Phuket and also previously from Kuala Lumpar to Phuket. They're a great airline and the fares are very good.

As someone mentioned, Less is often More. A month seems like a long time but you'd be surprised the stuff you will miss by switching countries. You're really filling up that month with a lot of destinations.

Flights are typically very cheap and are a good way to make long hops. Busses and shuttles are other options as well as private drivers which are typically pretty cheap.

At a minimum, Mayo's Travel Medicine service recommended Hep A&B, and Typhoid as well as MMR and Tdap boosters for Vietnam.

Have a great trip. It'll be fantastic.

Another airline note. We’re excited to fly from Tokyo on Alaska Airlines new service to Seattle.

We appreciate all of your comments and suggestions. They will be helpful with our planning.

Loved Thailand! I think a stop in Singapore makes sense geographically. Japan should have more time maybe a separate trip?

Went to Thailand in 2012 and had to take typhoid pills and get Hepatitis A vaccine.

Not much to offer on airlines.