I spent multiple times in both HAN and SIN, and while I think Hanoi is the better and more interesting destination--and very unlike HCMC--I reluctantly think this is the one you should probably cancel this time around, and save it for later. Why?
1) I'd tend to take the cash refund now rather than a future credit--you never know when something happens in your life that makes a future credit tough to use/schedule. Cash back in your pocket is always good.

2) Hanoi in November can be subject to rain/flooding. I happened to be in Hanoi last year during the latter part of October, and headed south just before the heavens opened and parts of Hanoi were flooded for several days, causing a lot of disruptions.

If the ticket refund situation #1 were equal or if you are really positive you can use the future credit in the next year, I'd take on the risk of #2 and go to HAN. I do chuckle about your worry that you might not find enough to do in HAN--actually, it's the stretch in Singapore that you may find tough to fill. For most people, 2 days/nights in Singapore is enough if you are purely a tourist. Not to mention SIN is expensive. Maybe there is a short daytrip or overnight trip you could stick in there, if you decide to keep the SIN ticket.