That's a pretty good idea, Rick, and would make a really sweet banjo, but it would cost most than the $975 you'd pay for the Vega tenor and the new neck:
1. New headstock tuners and a fifth-string tuner (approx. $100 for a set of Gotoh tuners);
2. You'd need a luthier to fit the new neck to the Vega pot. He'd have to steam out the dowel stick from the Vega pot, install it in the new neck, and cut the heel to fit the Vega pot. I'm not sure of the cost for that, but it would have to be done by a good banjo luthier because that's precision work. It has to be done just right for the best sound and for low action.
In the end, it would probably cost a lot less than an original Vega five-string tubaphone. I like conversions because there is a real mojo to those old pots.