Not a skater myself, but I often see skaters on the flat plaza at the top of the lake in old Lenin Park (Le Duan Street). Hope this helps.

Hey Koinkoin, I skateboard and have just moved to Hanoi myself. I have seen/heard of a few spots around town, why don't we get together for a roll? How might I contact you? Laters.

Hey, I'm a photographer and wanted to grab some cool skating photos, so if you can do a bit more than the general ollie, kickflip & curb grind then I'd be down for photographing some cool jumps/handrails.
PS - I used to skate a while back, but inline.