Hi ErinLee, and welcome!

As for many items, there are areas with concentrations of bike shops in Hanoi. Two that I know of are on Bà Triệu at the level of Thống Nhất Park, and at the junction of Láng Hạ and Thái Hà streets.

I think you can get a new, but very flimsy un-geared Vietnamese bike for ~500,000 VND. That seems to be close to the minimum.

I don't know - are chinese bikes reputed to be better or cheaper than VN bikes?

I have some friend who has just bought a new motor bike, dont like used bike so they leave it at the corner .If you would like I recommend you .Anyway I would like to meet you as it is posible.