It is yet to be discovered what "fully powered" means with Rigor.
- At the Beginning, Gogeta was a significant Step stronger, so Vegetto should be as well, of Course.
- Later, Rigor's Chi/Strength grew more and more, overpowering Gogeta. Luckily, Rigor made the Mistake using all his Powers before One Hour of Hypersleep worn off. He could have gone even higher otherwise, likely.
- "Potara Fusion" is supposed/called to be even more powerful than the "Fusion Dance" - so Vegetto should be stronger than Gogeta, at least when they both get created from a Goku and Vegeta who are exactly as strong as their Counterparts.
So SSJ4 Vegetto should have been able to take Rigor on significantly longer. He is stronger at least One or a few Steps than Gogeta - and he doesn't defuse* into Two different People after a certain Length of Time. So Vegetto could in all Honesty taken Rigor on for a longer Span of Time, but we don't know if he could have defeated him.
In the Case of Doubt, i would say: Yes, Vegetto could have defeated him. Maybe even Gogeta, as well. Since when they are stronger than him at first, then they can inflict enough Damage over the first Minutes, that Rigor may end up to injured to finish them: maybe even enough to lose his Supersayan State because of it. This would ensure Vegetto a sure Win - and Gogeta, too.