You don’t have to wait for Ama to make suggestions for HCMC. For both Hanoi (I came in 3 days early) and HCMC (I gave myself an extra day) I searched for motorbike and vespa tours on Trip Advisor. I ended up doing an evening motorbike food tour in Hanoi, a daytime vespa tour in Hanoi, a women led vespa food tour in HCMC and, because there was no information from Ama about what they were doing on the free day in HCMC (which ended up offering an optional excursion to the Cu Chi Tunnels by bus), I planned an all-day tour by speedboat to the Cu Chi Tunnels, lunch, return to HCMC, and then concluding with a half day Vespa tour. I compared companies and ratings on Trip Advisor, checked out all the websites, and chose the ones I thought best for me.
As an aside, I found sitting behind someone on a vespa to be a lot more comfortable than a motorbike. The position was more comfortable, and I had somewhere to at least rest my heels - my whole foot wouldn’t fit - while riding. I had never ridden either before, but I am a cyclist (bicycle) which made balancing something I was used to.