I'd have to think that the farlang-acised Viet alphabet would make things easier.  

 

Are speaking/hearing Viet tones more difficult than contending with Thai tones ?

According to this website https://bilingualkidspot.com/2019/06/07/top-10-hardest-languages-to-learn-for-english-speakers/  Thai comes in at #7. 

In another list of 25, Vietnamese and Thai are pretty much thai'd.

https://list25.com/25-of-the-most-difficult-languages-to-learn-in-the-world/5/

 

I've learned to read and write Thai, albeit not very well.  I also cracked the code on tones.  The hours I spent with one on one with a Thai teacher doing tone exercises were brutal on the brain cells.  I could do one hour, max, and then had to call it quits.  But once I got it, I got it......now I'm frightened of losing that ability if I go a few days without reading/writing/speaking

It sounds like you are contemplating starting to learn either one. With that in mind, I think it's important to look at your motivators and how drawn you are to either culture and people of these two countries. 

For me, I could never imagine myself speaking Vietnamese because it sounds so wrong to my ears (if that makes any sense). I like the rhythm and sound of Thai language much more and I think that was a big part in keeping me motivated working on it. 

 

Obviously, you might see it the other way around. I would choose accordingly.

The English alphabet, which Vietnamese is written in , makes it much easier to learn for people familiar with the ABC

Vietnamese may be written in English characters, but don't be fooled, it is a difficult language to master. 

Once you can read Thai and understand the tonal rules, you are really on the way to speaking clearly.

And it's not difficult if you're motivated, believe me. Phonetic exceptions to spelling patterns are miniscule compared to English.

Forget trying to learn Thai using English letters, it's a different game.

i can't answer regarding learning Vietnamese, but I'd guess using English letters would be a serious impediment.

2 hours ago, kokesaat said:

According to this website https://bilingualkidspot.com/2019/06/07/top-10-hardest-languages-to-learn-for-english-speakers/  Thai comes in at #7. 

In another list of 25, Vietnamese and Thai are pretty much thai'd.

https://list25.com/25-of-the-most-difficult-languages-to-learn-in-the-world/5/

 

I've learned to read and write Thai, albeit not very well.  I also cracked the code on tones.  The hours I spent with one on one with a Thai teacher doing tone exercises were brutal on the brain cells.  I could do one hour, max, and then had to call it quits.  But once I got it, I got it......now I'm frightened of losing that ability if I go a few days without reading/writing/speaking

I would agree that Thai and Vietnamese are roughly of the same order of difficulty for a native English speaker to learn.  I would also agree with your first source about those languages being difficult, too.  But I think your second source (25 most difficult languages to learn) is a real mishmash of information and misinformation.

The Scandinavian Languages (Danish, Swedish, and Norwegian) are all relatively easy for English speakers to learn.  Afrikaans is also not a difficult language for English speakers to learn.  Indonesian and Malaysian are relatively easy languages to learn as well.  So it's jarring to see some of these languages mentioned by the second source as being among the most difficult to learn.

Everyone seems to agree that Chinese, Arabic, Japanese, and Korean, are exceptionally difficult for English speakers to learn.

Thai and Vietnamese both seem to be relatively difficult for English speakers and occupy a very large middle group in terms of difficulty.  Tonality makes both of these languages a little more difficult than the rest of the languages in this middle group.

Here's a much better compilation from the Foreign Service Institute of the US State Department of foreign language acquisition difficulty for English speakers:

https://www.atlasandboots.com/foreign-service-institute-language-difficulty/

Most Chinese Malaysian like me can speak Mandarin (4 tones) and some also can speak Cantonese (6 to 9 tones) too like me so learning other tonal languages is easier but I find Vietnamese to be a pain to the ears when listened to even though 60% of Vietnamese vocabulary came from the Chinese language. To me, Thai is more pleasant to listen to. And because the Malay language has many Sanskrit words in it, learning Thai for the Chinese Malaysian like me is easier. Thai's style of placing the adjective after the noun is similar to Malay, example: rot fai (thai) = kereta api (malay) = car fire = train.