my vin d'orange came out bitter this year... any way to salvage it?

I'd blame the grapefruit, too! I don' t know that it's salvageable as vin d'orange. Typical things that will help reduce the perception of bitterness are increased sugar, salt, or fat. Obviously you are not adding fat to the mix. You could try adding extra sugar and/or a little small to a trial portion to see how it works. I'd also consider researching bitters to see if you could convert it to homemade bitters instead. http://spiritsandcocktails.wordpress.com/2008/04/22/how-to-make-bitters/