Hey fellow Vietnamese-American here. My Vietnamese stuttering way worse than my English for a lot of the reasons you mention. Syllable timed speech will help if your Vietnamese is fluent enough. I find that it works but when you're not prepared it's difficult to get out a "cảm ơn" without being stuck on the C part. Best advice is to just speak it slow especially when you sense a block coming up. If you can use "dạ" as a great filler word and really makes you sound polite! Being nervous just makes it worse and trying to speak at a fast pace without a steady rhythm will mess you up. Worst case switch to a singing tone. Vietnamese can be wonderfully spoken in a song format. Just start at a higher base pitch and act like you're impersonating someone with a higher pitch. I find that using my creative side alleviates the blocks at time. Good luck! Chúc may mắn bạn!