Dear Penfold,
I know that you are Chinese but I am not sure what dialect you speak. If you speak Cantanese or Mandarin, there are some similarity in spoken vietnamese with those 2 dialects. I would think above 80% vietnamese vocab are Vietmaese Chinese which means vietnamese spoken words originated from spoken Chinese.
Anyway, I can share my experience of improving a second language is to read newspaper and listen to radio ... and make vietnamese friends.
Cheers
Camtieu