Q.Can I mulch with leaves?
I have 10 flowerbeds and lots of trees – have a vacuum/leaf grinder. Can I use this to create mulch for the flower beds? If so, how thick should I mulch?

Ground up leaves make a superb mulch. You'll want to mulch twice a year, less (a couple inches) in spring, and 4 - 6 inches in fall. Here's some general mulch information: https://www.gardeningknowhow.com/garden-how-to/info/whats-the-best-mulch-for-my-garden.htm
and a couple of articles on leaf mulch:
http://www.extension.umn.edu/garden/yard-garden/soils/options-for-disposing-of-leaves/