I've been living in Vietnam for years now and watched many foreigners struggle to learn the language, which depends even more on tones than Chinese. Tonal languages are very unforgiving, and the differences between tones are very subtle in common conversation. Even when Vietnamese completely mangle the grammar and pronunciation of English you can usually understand what they're trying to say. But if you don't absolutely nail the tones and pronunciation of Vietnamese all you'll get are blank stares back in response. That is, if you haven't accidentally said something wrong or vulgar by mistake.

The saving grace of Vietnamese is that the grammar is trivially simple compared to English. No plurals, tenses, cases, verb conjugation etc. And it uses the Latin alphabet so reading and writing it is vastly easier than Chinese.