When I came back to Canada I changed everything directly from VND to CAD because I compare the value of VND against USD. So had I changed to USD then CAD it would not have made a difference.
I would think that the gold/Jewelry shops use the Vietcom Bank figures for exchanging money.
Ngân hàng th??ng m?i c? ph?n Ngo?i th??ng Vi?t Nam - Vietcombank
Check the rates on here and see what works for you.
Not sure how they value of CAD against USD is affecting its value in Vietnam but we generally trade USD upwards of 22,000vnd now.
I think I also poster before about a popular place to change money in the heart of HCMC on Dong Khoi street, across from Sheraton. A lot of different people use this place, or can try a gold store and see if you get any better but in the end unless you are changing at least a few grand then the savings will be small.