Try Hoan Kiem Lake (Hс╗У Ho├аn Kiс║┐m) near the Huc Bridge, the washrooms at night.
Just don't let them take you to a karaoke bar....it's almost always a scam.

I've heard there are also rent boys at Lenin Park...but haven't needed to go as there's always plenty at Hoan Kiem.

Surfcrest