1usd = 22,900vnd was the exchange rate last time I did the ATM withdrawal using Schwab Debit Card on Monday. Charge refunded was 1.94usd
What bank here?
Also, don't forget: when figuring out the exchange rate, you have to include the ATM fee, as if you received it as cash.
Of course, you receive the money and the bank gets the fee but BOTH amounts are then subject to the exchange rate from dollars to dong.
Charles Schwab Debit card & like I said before the ATM fee of 1.94 usd was reimbursed by Schwab to my account. So to make it simple:-
I withdrew 3 mil vnd at the Sacombank ATM using my Schwab Debit card. My Schwab account was debited 131.02 usd for the transaction but then the ATM transaction surcharge of 1.94 usd was credited back to my account by Schwab. This card is linked to a stock trading account at Schwab & the cash is held in the Schwab Liquid assets fund & as such earns a healthy dividend paid monthly. Hows that then?? 
So much fluff & stuff, yet you still aren't providing all of the DATA so we can all compare experiences correctly...but I don't think that's your fault.
As I recall, the last time I used a Sacombank ATM, the ATM fee was NOT listed on the receipt.
However, the ATM fee WAS included in the $131.02 charge to your Schwab account.
Since you received an immediate ATM rebate (unlike Schwab One accounts which must wait for monthly rebates of the aggregate of ATM fees) then you can simply subtract the ATM fee ($1.94) from the total account debit ($131.02) to get the currency charge for ₫3,000,000 ($129.08).
Divide 3,000,000 by 129.08 to get
23,241.400681747 or ₫23,241 which was the approximate exchange rate you received, NOT ₫22,900 as you previously stated.
It's important that we don't mislead the casual reader.
Also, if you multiply 23,241.400681747 by 1.92 (the rebated fee amount) you get:
45,088.31732259 or ₫45,088 which is the APPROXIMATE amount of your ATM fee.
However, somewhere in your two numbers is the hidden exchange fee.
Someone else likely knows how to get to that, but I'm guessing your Sacombank fee was ₫45,000 VNĐ and your "hidden" foreign exchange fee is represented by the 88 dong remainder from your ATM fee, which currently = about 0.0038 USD (or 38/100 of one cent)
Whatever percentage that amount represents is the same percentage that could be applied to your $129.08 to get the rest of the hidden exchange fee.
I'm posting this all here because in previous discussions, some hater always comes along and accuses Schwab of charging higher exchange fees than Transferwise or some other gimmick, when (as we both know) Schwab gets us about as close to the true exchange rate as possible.
Cheers!