In this book, Nguyẽn Đình Hoà says at the bottom of page 24 that the /w/ sound is spelled -o following a or e, and is spelled u everywhere else. However, he gives as an example "máu (blood)" which breaks that rule, and the word "áo" also breaks that rule.

Is there a clear rule? If so what is it? There is something in his explanation that I am not understanding. I would have emailed me but he passed away in 2005.