South Korea and Thailand- do both in one trip?

I spent two full weeks in Thailand and it felt like a minimum. Enough, but a minimum.

We went to Thailand, Cambodia and Vietnam after a three week trip in China since we were in that part of the world. Two weeks in not enough time to add other countries.

Two weeks for Korea is pretty ideal. Trying to add any other country in that time frame is IMO a bad idea...you will shortchange Korea, and only see, maybe, Bangkok for a few days. Adding Viet Nam on top of Thailand to this trip length, by logical extension, clearly makes zero sense.

If the timing works out you could include a few days in Tokyo in early April to see the Cherry blossoms, then fly over to Korea for the rest of your time. There are direct flights between the two city airports (Haneda in Tokyo and Kimpo in Seoul) that make it much easier to get between the two cities versus using the international airports, and booking an open jaw ticket (flying into Tokyo and home from Seoul) would further simplify your logistics.

My last visit to that side of the world was December 2019. I had 2 1/2 weeks which was going to be devoted to Phuket but decided on a stopover in Kuala Lumpur there and back for a few days.

The few days that were spent in Kuala Lumpur were quite full and certainly didn't allow for any further exploration. Phuket was a relaxing beach holiday so only a few local trips however I have been to Thailand previously.

I've also spent two weeks in Vietnam with a stopover in Singapore.

Both Thailand and Vietnam can occupy two weeks by themselves, I wouldn't try and add on South Korea unless you want a brief city stopover.

I've lived in both Seoul and Bangkok; yes, you can visit both countries in two weeks but it depends on where you want to go and what you want to get out of it. Korea is small and easy to travel around. Thailand is much bigger and travel is a bit slower but not too challenging. I would prioritize Chiang Mai/ northern Thailand and the islands. The best islands tend to be a bit harder to get to. April is Thailand will be Songkran- busy and HOT.

I have NOT been to S. Korea or Vietnam... but have visited Thailand twice. Thailand can easily fill up 2 weeks and more. But it depends on what you're looking to do.

Like if the question was, "Can I do Seoul & Bangkok in 1 2-week trip?" then yes. But in Thailand for instance, there's always a few days trips you can make out of Bangkok (e.g., Ayutthaya), then two other big/popular cities are Chiang Mai & Phuket. Chiang Mai is great for outdoorsy people. But also historical people -- you can plan day trips out to Chiang Rai, the Golden Triangle, etc. Phuket is a beach city, so again you could easily spend quite a number of days there. I personally love Phuket as a base for scuba diving -- diving in the Andaman Sea is one of my favorite places to scuba dive.