I spent 10 days in both of those cities this past September.
Skating wasn't a huge focus, but from what I seemed to gauge from my experiences there, Hanoi seemed to be solid for skating. I came across a few open areas that have some ledges and huge areas for flat in Hanoi - being Lenin Plaza / Park (near the HCM Mausoleum) and another one being on the east side of the Ho Hoan Kiem lake - Google street view might be your friend in locating it (recall seeing Pier 7-esque manual pads, but of marble). I think there is a "Skate: Hanoi" on the Ride Channel that has both of these spots if you want a visual as well. Seemed like a decent amount of people skating in the early evenings passing by these spots.
I didn't see anyone skating when I was in Ho Chi Minh, so no dice on there from my end. Hopefully someone else has some any input there.