The Lowepro ones are very good, I've got one of the "Slingshot" range - check them out here:
http://products.lowepro.com/catalog/Sling-bags,4.htm
They come with a raincover that folds into the bottom of the bag.
I've seen a few in one of the bag shops at the top of Hoan Kiem Lake, they looked genuine (with the proper labels etc) but I'm not sure.
I have seen genuine ones in a small camera store on Trang Thi St. I don't know the address but it's on the corner opposite the hospital, on the north side of the road.
The Crumpler ones you can buy here (fakes) are pretty good too (I have one and it hasn't let me down), but they don't have raincovers.
The shops around the north of HK lake are definitely the best bet.
There's a couple of shops opposite the Bo Ho bust stop with genuine Crumpler bags in a good range of sizes to suit your setup and colours to match whatever outfit you like to wear.
I bought one about 8 months ago for my Nikon D80 which goes literally everywhere with me and it's been great - copped the heaviest downpours VN has to offer and stayed dry, and had a hell of a beating to boot, but still looks great. I couldn't recommend one highly enough.
