I've a black Boxster as well, and when this happened to me I quickly took out my trusty Porter Cable random orbital polisher and some Menzerna intensive polish and it came right out.
Keeping a fresh coat of carnauba wax on top of whatever sealant you are using will also help water spots from sticking to your paint.
I'm betting the vinegar will strip your finish of all wax and sealants, but I have no personal experience with this. Hopefully, one of the highly knowledgeable forum members can chime in and help you with a non-buffing remedy!
smith... i had a similar experience not too long ago...(search under "water spots"), ...and had the same solution offerred up from a well-known local detailer....vingear/water-- i didn't have the nerve to use it cuz the culprits that "spotted" my car...paid to have it detailed.
If you do use the vinegar--- make sure you post you your experience with it.
It sure seems that out here in So. Cal. a black car get "dusty" 15 minutes after your done cleaning.....sure makes me envy that Arctic Silver!!!