Once you’ve learned how to pronounce Vietnamese and have a handle on the relatively simple-to-learn grammar, learning Vietnamese is mostly a matter of picking up vocabulary. What you’ll learn in books is the the tip of the iceberg, and may even include a lot of words that aren’t so frequently used.
For English, there is the General Service List (GSL) containing the 2000+ (2284) most important (by some definition of “general service”) words of English. There are shorter and longer lists such as this Basic Englishword list of 850 words.
No such GSL exists for Vietnamese but there are a few word lists available.
The Corpora of Vietnamese Texts (CVT) is a body of work containing a million or so words and each word is broken down by frequency of use in the Vietnamese Word Frequency List. The list has some 14000 mostly Vietnamese words in the order of how common they are found with the number of times and percentage listed by the word. However, the bottom of the list is heavily polluted with non-Vietnamese words that were used just once or twice throughout all the text.
Another set of words is found here. The file contains four lists, from 11000 words to 74000 words. The lists contain Vietnamized loan words but written using only the Vietnamese alphabet. Words are ordered alphabetically and no word frequencies are listed.
Neither list provides an interface for using the words. They are purely just lists and you’ll have to find ways to use them, whether making flash cards or importing into flash card software. You’ll also want to translate the words before dumping into software like Anki or Mnemosyne/Mnemogogo.