Some of the people at my barn vacuum their horses. I have not done it to mine. I never really looked that close at the vacuum they use but it does seem to be something like a shop vac.
One thing I have to say is when they get their horses spiffed up they look and feel like pure velvet!!
I do not think my gelding would be to tolerant of it. Maybe if he got used to it. My mare on the other hand would probably do okay.
Something to think about in the future!