Hmm good question. I'm going to say no you can't redirect damage to Dethvir after you've used Ring of Invicibility. Since damage can't be dealt to Dethvir, I don't see how you can redirect it to him using his effect.
Here's the last part to Dethvir's text. "If another hero or ally in your party would be dealt damage, you may have it be dealt to Dethvir instead." I don't see how you can do this effect with Ring of Invicibility.
This scenario is much different than if you played United Front. (which only prevents damage).
~DF
Question on Ritual of Summoning...
Conrad Hex:
No. You also can't bring in a Pokemon card, the 6 of diamonds or a Christmas card from your Aunt.
LOL!