Gas leaking from the gas cap is far more common on Vulcans than many may think. The problem is always going to be related to one of four things:
1) Overfilling of gas tank - fill to the bottom screen only.
2) Clogged venting on the gas cap
3) Blocked breather hose
4) Cap seal problems
The first three are most common causes.
- RandallKS VDR#316 (Delphi Vulcan Drifter Forum)
The underside of the cap actually has a fairly complex system for venting, including a number gaskets. It's really easy to remove the cap (2 screws). Before removal, cover the tank hole with some painters tape or masking tape. You don't want a screw to fall in there. Check the large rubber gasket to see if it's dry and cracked. If that looks good, next check the small rubber breather gasket in the base of the cap to make sure it isn't clogged. If both of those are okay you may need to disassemble the cap completely and try blowing a little air (use a straw - NOT compressed air) into the tiny vent holes.
Another cause could be a clogged or kinked breather hose at the rear of the tank. On the California models it's a bit hard to get to. I had to remove the seat, the rear tank bolt, the speedo housing and the two front tank bolts, then lift the tank. On other models, maybe it's accessible by just removing the seat. Once you can get at the hose, see if it's clogged or kinked.