Reddit mentions of A Vietnamese Reference Grammar (Mon-Khmer Studies)

I use Duolingo. It's the only app that I've found really useful. Clozemaster has Vietnamese, but it's... pretty worthless. Memrise is nice, but only for memorizing vocabulary. Really, I've found practice with native speakers is an absolute necessity, because it's impossible to pronounce things correctly without it. Even with a significantly better vocabulary than when I started, it frequently takes a couple of restatements of what I'm trying to say before my meaning is clear.

For understanding more about the language, I recommend the book A Vietnamese Reference Grammar. The "scriptorium method" is also quite good, combined with the inexpensive Tiếng Việt study books intended for Vietnamese schoolchildren that you can find in any Saigon bookstore.