Sure the alphabet is a big plus for someone who doesn't know any Chinese (nor Korean or Japanese). In the contrary the old writing system (chữ nôm) could be extremely useful for learning vocabulary given how many words are from Chinese origin.

I was working with a Vietnamese-French dictionary (Đại Nam quốc âm tự vị hợp giải Đại Pháp quốc âm 大南國音字彙合解大法國音) and I was struck by how many words I can understand without reading at the definition because they were writing in Chinese characters; of course when it was nôm character this didn't happen. The reading of this words also weren't that surprising compared to Japanese On readings or Mandarin Chinese.