There's one money changer near to Caravelle Hotel, just opposite LV Store, that's famous for changing VND in USD. They used to be at the LV store but was kanna forced to shift opposite when the building got torn down. That money changer has been around for quite a long time and they have large quantities of USD. I remember once in 2003 when i saw a Viet Kieu changed VND to USD for amount of nearly US$60K. Changing VND to USD is never a big problem in HCMC, but its the bringing out of TSN Airport customs that's the problem. If you have offices in both Singapore and Vietnam, best is to produce legal necessary documents and do proper Bank TT. But look at it, who will want to bring cash out if he can do it the legal way?