Yes, I withdrew it in USD, wired USD to a bank in Vietnam, and the converted it to VND here. (You probably don’t want to withdraw it in VND directly anyway. The interest rate that IB charges you varies depending on the currency you take it in. If you took it in VND they’d probably charge a higher interest rate.)
I don’t think you’ll lose as much as you fear, though. A quick check on xe.com says that the live mid-market rate is 22,773 right now. And Vietcombank will convert at 22,745. That’s, what?, 0.1%?