Have you ever made a hollow bead? It's the exact same method, you just make it at the end of the mandrel, sealing off the tip with glass to form the bottom. I also make a wider "bead" at the other end to act as the neck of the vessel. If you haven't made a hollow bead before I'm sure there are probably some great tuts (better than I can give on a Monday afternoon) with pics somewhere on here.
__________________ High-temp, military grade 1/4" hollow mandrels are now available in my Etsy shop!
I have 12" and a limited number of 9" ready to go! BULK PRICING AVAILABLE
Also Available in my Etsy shop: my Electroforming for Everyone booklet.