What is the hardest sound to learn in English

in general:
bitch vs. beach
full vs. fool
can vs. can't for some people
And for me personally the ul (as in cultural) and some combinations of a consonant followed by an r at the beginning of the word (e.g. thrushing).

for American accent:
strong r (boston accent is a notable exception here)

the t: if starting the word, it's pronounced a lot more fully than the vietnamese "t"
if in the middle of the word, it could be a "d", or held, or completely silent.
e.g. written, water

I'm not an English learner, but I am an American English listener (like you). In the US, the thing that I notice the most is that native Vietnamese speakers don't pronounce the whole word, especially when there are consonants after a vowel at the end of the word. I think the hardest words are words with a vowel, consonant and then ending in a “t” (e.g. tonight=toni, want=wan, etc.), but there are other words like “vegetable=vegetaba” where people just don't pronounce the end of the word. Most of the time you can still understand what is being said, but it is a common pronunciation mistake with native Vietnamese speakers.