(1) Pimsleur podcasts: https://www.pimsleur.com/learn-vietnamese/all-products?coupon_code=pla

(2) Duolingo app

(I am a passive leaner, so Pimsleur and Duolingo work incredibly well to me. I spend 5-10 minutes per day on Duolinggo. I also play the Pimsleur podcasts on my phone while I'm running, cooking, showering,...And that's how I master my Chinese and Spanish)

(3) Quizlet app: It's very useful to remember words.