2 weeks in SouthEast Asia. Need suggestions- Where to go- etc...

Might depend on what you WANT to do. You could head north to Chiang Mai (and maybe Chiang Rai) afterwards. More mountains after your beaches and large city. I like Chiang Mai.

But if flights to Vietnam fit your budget, go for it. Looks like you could fly rt to Hanoi for under $200. And 2 weeks is a decent amount of time. I haven’t been to Cambodia yet. And yes, Japan will be more expensive. Maybe save it till you can spend more time.

Perspective. Tokyo is a 7 hour flight from Bangkok.
This is as if you were visiting NYC and New England and that perhaps you might fly to London also. And then, back to NYC to return home.

Look into Chiang Mai and Chiang Rai as suggested above. I also thought Kanchanaburi in Thailand was spectacularly beautiful.

Also research Siem Reap - base for Angkor Wat and multiple other fascinating temples and sites.

The above locations can be done budget (I confess I didn’t do the places on a shoe string, but many do.). Fascinating places - I’ve had to make return visits!

I don’t know Viet Nam. Japan is more expensive than your other considerations as you suspected.

I really like Chiang Mai. However, there was horrendous rain in early Oct in Northern Thailand that resulted in massive flooding. Go only if your research shows that your destinations have recovered.

Hanoi and Ha Long Bay are more interesting than HCMC, which is mainly a commercial hub. The weather in Hanoi is generally cooler, though it was also affected by a big storm in Sept.

Anyway, bring mosquito sprays with you. Local products are usually Deet-based.

I have not been to Ankor Wat and therefore cannot comment.

You can also focus on Vietnam only (Hanoi, HCMC and their surrounding areas) with the remaining time.

Consider Singapore. Though it can be an expensive destination for tourists, you can do it on a budget if you "go local" by steering clear of the luxury hotels and book your accommodation in one of the ethnic neighborhoods (China Town, Little India, etc.), eat at any of the excellent food stalls located in virtually every commercial building, and rely on the SMRT (the local metro) to get around to see the sites ... of which there are many - the place does have a rich and fascinating history, and everyone speaks English (more or less).
It's a cheap 2.5 hour direct flight from Bangkok.

You can’t go wrong with either Cambodia or Vietnam. Both are budget friendly with excellent food choices. It looks like you will have beach time in Thailand so I would skip HaLong bay. Maybe combine Hanoi with Ninh Binh. If you choose Cambodia the temples by Siem Reap are amazing. Phnom Penh seems to get skipped by most travelers but I really enjoyed spending time there as well.

I've spent time during the last two summers in Siem Reap. (I went back this summer because I didn't get to as many temples as I wanted to in 2023.) I highly recommend the city and the Angkor archaeological complex. It's just amazing. Siem Reap is not very expensive and hiring a guide is, in my opinion, the way to go because you can see a lot in a brief space of time. I'd recommend at least 4 or 5 days.

More grist for your mill:. Maybe they're not your thing, but Hue in Vietnam has great food and fascinating history and Taipei, and Taiwan in general, turned out to be way cooler than I thought it would be when we visited a few weeks ago.