Bopomofo
I think I like it better for representing all the sounds because it makes harder distinctions between things like the two -i's. It's also kinda like kana is in Japanese. Pinyin is leading me to trying to pronounce things too much like English where the sounds are sometimes not that similar. :-/
All popos are mofos.
On a serious note, there are many languages where trying to implement an alphabet was a disaster. Vietnamese would probably be way better off with a logography like that of Chinese.
AFAIK, it used to have its own logograph that fell out of favor. See here