I've done 20ish dives out of Nha Trang. The diving is cheap, figure $35 for 2 dives and lunch, hotel pickup yada yada yada. The instructional staff at Rainbow divers and Sailing club divers are very good. They run PADI IDC's together. Your timing is poor however. The viz won't be good at that time of the year, although some years it can be ok. Rainbow divers runs op's out of Con Dao island and Phu Quoc island. I'll be going to Phu Quoc at the end of Dec. I'll try to post a trip report at that time.
Nha Trang can be outstanding when the weather is right. The outer islands have some very nice dives. The standard dives that the ops go to when the weather/viz are poor result in a lot of negative reviews. One note of caution, you can get dives in Nha Trang very cheap, I have seen several of the ops not mentioned do things that were unsafe IMHO. One was recently removed from PADI.
Second hand I have heard about the other dive sites in Vietnam. Check w/ the folks at Rainbow divers about conditions in Phu Quoc and Con Dao while you are there. They gave me thier honest opinion about the conditions the first time I went to Nha Trang and convinced me to come back at a later date. I have done several courses w/ Sailing Club divers, ask for Simon White (SC manager) to get another honest opinion about the days conditions. If the conditions are right try the speedboat trip to one of the outer islands costs a bit more, but almost no ever dives there.
A disclaimer-- I don't have any affiliation w/ any dive op in Vietnam, but I have family, In-Laws, that bring me there every 18 months or so. I know the folks at the Saiing Club divers outside the shop, have had beers and meals w/ them, so I might be biased.