I'm not sure what size it is but most spark plugs are the same in vehicles. Your best bet is to buy a spark plug socket. Then you have the right size and the little rubber oring they put inside of it to hold the plug. Saved me on more than one occasion!!
I'm about 95% sure that a regular 5/8ths spark plug socket fits the Vulcan just fine. I know a 3/4 is too big. I used one out of a cheap Taiwan made set as well. You have to make sure that the rubber part actually locks onto the plug. Sometimes on a newer socket it can take a bit of force.
You may also have to clean around the plug with a pick to get some of the dirt that can pack in around the plug. Some of them can be tight clearance around the plug (cylinder 4 on my 95 was real tight clearance and I ended up using a thin wall on it).
If you do pick out some dirt, it's a good idea to blow the residue out with air, carb cleaner or something of that nature.
I agree, to save yourself trouble, buy a spark plug socket. They are normally thin walled, they are deep enough for spark plugs, and they have a rubber cushion, which holds and protects the plug as you install it, so that the risk of damaging the plug is minimized. They also have a 3/4" hex head, which is useful in some tight situations.