Where's the audio?
This won't work without the audio. I haven't the slightest clue how to pronounce all those different diacritical marks on the vowels.
The Vietnamese course uses recorded audio, as opposed to a text-to-speech (TTS) software, which means that there are quite a few sentences that don't have audio, and none of the individual words (the select-a-picture exercises) have audio. But as long as you have sound enabled in your settings, you should still hear the audio for Vietnamese sentences that have a speaker icon next to them.
If you're struggling with the pronunciation of individual words, I recommend the website Forvo, which has native recordings of thousands of words in a huge number of languages, including Vietnamese. All you have to do is type in the word you're looking for and there should be at least one example of a native speaker reading it out; if not, you can create an account and "request" the word to be added.