For me the biggest risk is that the notes are counterfeit.

Yes, there might be a risk depends on what shop you want to buy USD from. Some are very reliable, some not.
What's the name of the shop you visited?

No the risk is limited and anyway if you get some counterfeit money you can always use it. I cannot make a different between fake and not and I am sure I got some but never had any problem from someone telling me that I carried counterfeit dollars, actually the rate yesterday was 16,780 so the most reputable place is close to the hang da market I do not have th street name with me but it is a small street in angle where they sells as well motorbikes seats you will find the shop there.

The gold shop right next to Victory CD/DVD shop on Hang Bac has always been good to me in the past. He always seems to have Dollars and Bhat, maybe other currencies too.

Thanks for all the answers. I went to Kim Linh shop at 67 Hang Bac and was reassured to find the 2nd floor exchange packed with Hanoians changing dong in all directions.