Learning Vietnamese is likely learning foreign language, so it’s useful to use or listen Vietnamese as much as possible……… you chose a good way, watching films subbed Vietnamese, it showed that you have experimental in learning foreign language. .
You can’t sit down, read from a Vietnamese-English dictionary and remember, since it’s boring, I can feel that :P.
So why don’t you go out and make a friend with Vietnamese who is good at English. It’s not the best way ,but I think it is better than reading from a Vietnamese-English dictionary.