Partly because it takes a while to travel between places on Bali I think this is true. I spent a couple of weeks between Bali and Yogayakarta. I definitely felt like the surface was pretty well scratched for me. Depends what you are interested in and what you like to do.

I liked Ubud way more than Nusa Dua, Seminyak, etc. But I don't really like beaches much so that probably explains that to a large degree. The beach places are more crowded-feeling, more traffic, etc.

Central Vietnam has some nice beaches. So if you're going to Vietnam anyway it's easy to combine that with Hanoi. I don't know the weather that time of year. I was in Hanoi and then Hue and Hoi An in November a few years ago and it was very rainy in central Vietnam. I didn't do enough research into the weather. Hanoi and the north was nice though. I don't know about May but it seems like the weather websites should be able to give you averages and climate info.