I am sorry that I cannot suggest anywhere near in Hanoi, or in Vietnam for that matter. But if you have time to travel, try the Horseshoe Point Riding Academy near Pattaya. We went there a couple years ago so my son could learn how to ride and it was very good. I believe they have over 100 horses and a variety of different riding areas. The instructors were very good. It is a bit isolated and you are stuck eating in the hotel, but for riding it is very nice.
