Whoa! Vendettas, if made official, would have nothing to do with official challenges. You offer the series. Your opponent is free to accept or decline -- though if he accepts, he does have to play all three games. Vendettas are for raising the stakes, for playing out a longer contest with someone. Challenges are different. Challenges exist so you can't run from anyone.
What's a vendetta? It comes from the old Descent ladder, the IDL. It's a three game series you play with another pilot. Where you play is up to you guys, though a lot of DCL vendettas look like "I pick, you pick, random". On DCL, there is no official support for them. No rules, no tracking on the site, nothing. It's just pilots who remember IDL doing what they used to do on IDL.
On IDL, at least, vendettas did a couple special things that you couldn't do just by playing a series of matches. The ladder enforced that you had to finish the series -- you couldn't play anyone else until you'd finished your three matches. There were medals and stats that keyed off of vendettas. And change in ranks keyed off of the result of the series as a whole. So you could lose the first game but win the next two, and keep your rank.
The question is whether we should support them in some way, and if so, how.
I, personally, have liked the freeform nature of them so far. But they're common enough that stats would be nice. So maybe it's time.