How can you cut a regular seat for a 750 vulcan too make it shorter?

You can remove the vinyl cover, an use an electric meat carver to cut some of the foam cushion out of the seat. You can try to reinstall the cover, but will probably have to send it to an upholstery shop to get it recovered. There are some other things you can do like adjust the rear shocks to the lowest setting, mabey get some shorter shocks, depending on fender to wheel clearence.