Evidence-based answer · Last updated · How it’s made
There's no source here that studies people learning Vietnamese as a foreign language directly, so this answer draws mainly on general second-language research (much of it on vocabulary, pronunciation, and tone learning) rather than Vietnamese-specific studies. The clearest, most consistent recommendation from these sources is to space out and actively retest your vocabulary and sound practice rather than cramming, and — because Vietnamese is a tonal language — to give early, dedicated attention to hearing and producing its tones and prosody, following methods shown to work for tonal languages like Mandarin.
Asked as someone learning Vietnamese (beta). Findings drawn from Vietnamese-specific research are framed for Vietnamese learners and may not hold for other languages; where the evidence is general second-language research, the report says so.
None of the sources report an experiment or study of people learning Vietnamese itself (its grammar, vocabulary, or tones). One source surveys Vietnamese students' beliefs about learning English, and another mentions Vietnamese students using mobile apps to learn English, and a book review briefly mentions a Vietnamese cultural picture-book framework — but none teach Vietnamese as the target language. So specific claims about Vietnamese have to be inferred cautiously from general second-language findings and from what we know about tonal languages more broadly.9,11,16
Across several studies, spreading out review sessions over days or weeks produced stronger long-term vocabulary retention than massing all the study into one sitting or cramming right before a test. In one classroom study, students who reviewed words with retrieval practice spread out over 17 days (with the gaps between reviews growing longer each time) improved far more than students who only studied at the start and end of the period. Another study using a notebook-based review method found that spacing reviews two weeks apart nearly doubled the improvement compared to no spaced review at all.2,5,6,7
In plain terms
Example
Simply repeating a word over and over to link it to its translation is generally weak; vocabulary training works much better when it also forces the learner to actively engage with what the word means, such as generating an example, retrieving it from memory, or encountering it spaced out over time.1,2
A study of foreign-language students found that those who used a computer-based spaced flashcard program learned more vocabulary than those taught with only traditional classroom methods. Separately, Vietnamese university students in a mobile-learning study reported finding language apps useful and easy to use for building vocabulary and other skills.4,11
Because languages like Mandarin use pitch to distinguish word meanings (not just to convey emotion or emphasis, as in English), learners whose native language doesn't work this way tend to find tones one of the hardest parts to master, and generic listening exposure alone is often not enough. Structured training that gives feedback tied to visual pitch information helped learners of Mandarin tones learn faster and perform better than plain right/wrong feedback.10,12,15
In plain terms
Example
How difficult a new sound system is to learn depends partly on the learner's native language: in one study, native speakers of a language that already used pitch in a related way (Japanese, which uses pitch-accent) picked up new tonal contrasts more easily than speakers of a tonal language whose specific tone categories worked differently (Cantonese), while speakers of a non-tonal, stress-based language (English) fell in between.10
Research on age and language learning suggests that starting a language later in life mainly makes it harder to achieve a native-like accent, while grammar and overall competence are less strongly tied to age of starting; this reflects both brain changes with age and the fact that a well-established first-language sound system can interfere with hearing new sounds accurately.3
In one study, learners who found vocabulary learning harder in general benefited more from an immersive virtual-reality learning context than from plain word-pair study, while already-successful learners did about equally well either way.8
In a survey of adult Vietnamese learners of English, motivation stayed high mainly because of practical needs like employment, even though many learners believed they personally lacked strong language-learning talent and that children learn languages better than adults.16
Now pick something to do it with
Each of these opens an overview of the apps, courses and immersion material worth a look for that — filtered to exactly what the advice above points you at: