I think the idea is something to consider.
Central vac versus individual vacs. I'd have to vote for individual vacs. I wouldn't want to fire up an entire system for an individual bay user, nor would I want the inverse of that and have a central system on all the time. Also, if the central vac would go down, then all the bays are down. Thus individual vacs seem to have more advantages.
Vac placement. Placement inside of the bay might have challenges such as noise for the customer and having to protect it from overspray. I would use the space outside on the back of the bay, such as between bays. That apron is hardly, if ever, walked on. So it wouldn't be in the way.
Tying up the bay. There would be a trade off, now someone would have to wait for a bay to free up from a vacuum customer so they could go in to wash. Most of the months I don't see this as being a problem, there's usually a free bay for someone to use. During the busy days, I tend to find customers waiting for a customer vacuuming while the bay is open. Of course, that only happens to those of us who have vacuum islands in front of the bays and not have a separate detail area with vacs.
Trash in the bay. I would assume this wouldn't be any different than trash in front of the bay at the vac island. The positives I would hope to see is that customers would have to walk on their trash while washing, and thus would pick it up. That might be wishful thinking. The other positive is that it would be easier to clean up the sticky, gooey stuff that always seems to be around the vac islands (gum, candy, mystery goo)
Pricing. This would be awesome to have the vacuum as an option on the meter box, for those with
credit card acceptors in the bay but not on the vacuum...you could now pick up
credit card vacuum customers. Unless you have an intelligent system which allows you to adjust the time depending on the selection on the meter box, the price of the vacuums would have to increase. Such as instead of 1min for .25 at the vacuum, it'll now be 30sec for 1min. Maybe it's time for a vacuum price increase?
This is an interesting option, something I may put on my wish list for research.