Memrise is a cool, free website (and app) that can help you memorize anything (in this case, Vietnamese vocabulary) in a fun, and sometimes competitive way.
There are quite a few Vietnamese vocabulary courses on Memrise. Before you study them you should check with a native speaker to make sure they are from the dialect of Vietnamese that you are interested in learning. Most of the vocabulary on Memrise is the southern dialect, so if you use it in the north you are going to get funny looks from people.
You can make your own levels to help remember your own vocabulary. I made a level in Memrise with a lot of Vietnamese words.
Check it out!