"I open her book."
Translation:Tôi mở quyển sách của cô ấy.
In my experience unless you get the whole tonal thing right (which is super hard when learning on the internet) they won't understand you anyway but they will appreciate your effort. I am learning Vietnamese but the only Vietnamese people who can understand me are my wife and her father. It seems to me that the Western ear (or at least my ear) struggles to differentiate tone and that tone is more important to the Vietnamese ear than any other part of the sound.
There's also the concept that I call 'reverse stage-fright', and that is how people may not have a lot of experience hearing someone speaking their language with an accent (read: YOU speaking Vietnamese). This is usually true with languages that aren't particularly popular for foreigners (especially non-Asians) to learn... like Vietnamese. That is, almost EVERY person they've ever heard speak Vietnamese was a native speaker -- or at least had native-like-phonology (like a child brought up partially bi-lingual).
So, when they do run into someone with a heavy American accent speaking Vietnamese (for example) it's very hard to understand -- even if you have gotten everything right (including the tone). Understanding an accented form of one's language takes practice and time. Sometimes the brain just can't believe what it's hearing and stops listening.
For some intuition on this, consider how many times you've probably heard a Spanish speaker speaking English with a heavy accent versus how many times you've heard a Kyrgyz speaker speaking English with a heavy accent (that is, of course, presuming you live somewhere where you run into Spanish speakers' accents often). You are more likely to understand the Spanish speaker since you have had a lot of experience tacitly learning the typical changes associated with that accent... that is, unless you live with a lot of English speaking Kyrgyzstani. :)
In BraveLeeFleas case, I would bet his wife and her father have heard his accent a lot more often than the other Vietnamese he's speaking with. They've learned HIS accent and therefore can understand him better.
In my experience, the only non-relative/friend of mine who ever understood my Vietnamese (besides my actual Vietnamese teacher) was a very young man who is 100% fluent in both languages and has spent his whole life living in a bilingual environment, interacting with just as many Vietnamese speakers and English speakers with a huge range of accents. Incidentally, I asked him and he thinks my pronunciation is good. Almost every other Vietnamese I try to speak with can't seem to understand a word... :(