Ben Nam Frankly speaking, I was in the another city since nothing much to do in Hanoi, you just need to submit the arrival stamp from Noi Bai Airport if you are planning to choose Embassy in Hanoi. Basically, you can do it.
However, I recommend you to be ready to go to Hanoi for in case of asking location proof but I think they wonβt ask you as long as you submit all necessary documents.