To answer one of your concerns mentioned above--no, if you live in an appartment complex, you can still buy and enjoy the bike without ruining it. You'll just need to buy a quality bike cover--and use it, religiously
You'll also need to keep the bike waxed/polished, to keep rust at bay. Other threads exist on this forum that explain in more detail how to do this.
If the odo reading has any credibility to it at all, the bike has had a life as a "garage queen" and thus has had a lot of "sit" time. Your inspection will need to look for signs that this "sit" time was also neglect time--or not.
As mentioned above, use a flashlight and a dentist's mirror to check the inside of the gas tank for rust. You don't want to replace or repair a rusted tank right from the "get-go".
Also, remove the right side cover, and shine a flashlight up under the bottom of the battery box. Look for signs that the battery has leaked acid onto the bike's frame. If acid has attacked the frame anywhere, this corrosion damage is virtually impossible to stop, or fix, from spreading/going deeper and the bike frame is "terminal"...so walk away from the bike.
With these inspection points out of the way, then the others mentioned above are then worth your time and effort. Look for dryrot (cracking) on the tires. New tires will run $250-$400 for the bike, depending on what brand/model you install, and who does it. If the tires are really new (as he says they are), they should look it (no cracking, lots of even tread).
Since the rear view mirrors are off the bike--ask to see them (the original mirrors will be shaped round
, at the glass--not square). Their condition (scratched in the right places--or not) will give you clues as to whether the bike has been dropped, or not. Make sure the license plate frame comes with the bike.
Also: ask what else comes with the bike: owner's manual, repair manual (Clymer, or Haynes), spare key(s), repair/maintenance receipts/records, etc.
If you can...ride your current bike over to see the "new" one (assuming it's not too far). Then...ask to ride the Vulcan. If he hesitates, or flat out won't let you....then there's a "red flag" for you....
Other threads exist here on the forum (use the "search" function in the above menus--also look in the "Vulcan Verses") that give other pointers and info on inspecting a used, possible VN750 purchase.
Hope this helps....