Cash always is a king but $50 to have $500 changed is too expensive. There is always different rate between Banks and hotels. But not quite different. Just VND 300-500/$1 commission. That means if you change $500 you will get a reduction of around $10 or lower. Just check the rate exchange with the bank through internet. If there is a big difference with hotel, then go to the banks which around you. That's best way.

1$=21000VND , about 1$ to 2$ for a cup of Coffee in Hanoi & Saigon