I suppose this is true of all tonal languages but Vietnamese is very unforgiving. Even when people really butcher the pronunciation and grammar of English I can still usually guess what they're trying to say. If you don't really nail the pronunciation and tones of Vietnamese people won't understand you at all. I've seen a lot of foreigners give up in frustration even after years of living here.
If you really want to learn to speak tiếng Việt you have to find a real teacher and drill the basics of tones and pronunciation. If you try to pick it up a phrase at a time chatting with people you'll never get it.
I also found out that after starting studying Chinese it is easier to me to understand Chinese people speaking Portuguese (my native language) because I understand a bit of the Chinese grammar and phrasal structures.
When other kids complained about our old Taiwanese teacher's English accent, I asked them simply: "let's say hypothetically we inverted the situation and found an exact parallel of your Chinese accent to her spoken English when Chinese comes out of your mouth, and you understand the accent is native. How hard will you laugh?"