I am a non-native speaker of Vietnamese. There are some pretty horrible mistakes you can make, honestly. I'll go through a few of them.

In Vietnamese, non-native speakers often confuse the word for 'mother in law' with the word for the male genitalia.

Also the word for "large" with the word for the female genitalia. So when ordering e.g. a large meal, if in doubt, just use the word for L ("luh") instead of lớn.

When referring to your mother-in-law, practice with your partner before the first meeting. Then, quickly ask for permission to call her "mother", which is easier for non-native speakers to pronounce.

Finally, the word for 'martial arts' and 'Vietnamese wife' differs only by a single tone. If you make the mistakes above, you may perhaps find out why that is -- usually via the medium of a flung sandal :P