money

Why bother? Use dollars until it is convenient to change.

Yes, you will be able to exchange money at the airport at that time.

The better plan is to use an ATM at the airport to withdraw dong from your bank account. The currency exchange fees at the airport are ridiculous. And using an ATM is always cheaper than a currency exchange, even if you avoid the airport ones.

I've been through Hanoi airport twice this year and the rates have been comparable to downtown Hanoi.
With regards to ATMs my advice is too use ones attached to banks that are open. I and a number of people I know have had their cards swallowed by ATMs in Hanoi. This happened to me at HSBC a few weeks ago. I just got a 'windows is shutting down' message mid transaction.

You can either exchange at the airport (small amount) and do more at Hanoi Old Quarter then or use some small dollar bills first and withdraw from ATM then

The currency exchange booths at airports in Vietnam are the exception to the rule! The exchange rates at airports are usually AWFUL! But, as above, they are competitive with bank or ATM rates in Vietnam. The best rates are from gold shops.

When I flew out of Ho chi Minh the exchange booth in departures closed about 11 pm. You need to take some dollars with you in case the same is true in Hanoi, and you miss it.

similar situation for me not so long back, I booked pre paid airport pick up and also my accommodation with Agoda.... it was easy, no need for dong until the next day I just had dollars that I did not use for back up..

It is very easy to exchange money at old Quater in Ha Noi. There are a lot of exchange agents and the rate is ok, do not worried.