Hey thanks for all the comments guys. I've been trying out a few different things with her, and I've had great games and some stinkers. I actually just played one of the most epic games of LoL I've ever had. I was Vayne on a team with an Anivia, Teemo, Yi, and Chogoth, and we were against I believe Jarvan, Sivir, Zilean, Warwick, and maybe it was Kassadin. I've been finding the biggest problem with Vayne is that it's tough to avoid feeding in the early game, since she's so squishy and really doesn't do a whole lot until she has some items. This game I was dying a lot early, but I was at least staying even (basically trading myself for Jarvan or Sivir, who I was laning against with Teemo). Now you can imagine my suprise when I realized that me breaking even was the best record on our team, and that Yi and Chogoth had no kills and had fed Zilean up to level 17 by the time I got to like 13.
Now, I mostly queue with a group of m friends, and they are all level 30s, but I'm only at 22, so basically I'm used to getting my ass kicked all the time, and I rarely find myself in situations where I have to be the one to take charge. This was just me by myself, so apparently it falls to me. Long story short, we were on our last legs (I even typed at one point that it might be time to surrender, but didn't actually start the vote), but we had still held them at bay for long enough (somehow), for me to get IE, Black Cleaver, and Phantom Dancer, at which point I could kill just about anything by glancing at it. Final build was Merc Treads, IE, BC, PD, Banshees Veil, and a BFSword that I was going to make into a Bloodthirster but we won the game before that was really an option. I ended up 25-17-6. Our team had a grand total of 41 kills, so I accounted for more than half.
Then of course the next game I played with her I went like 2-15-4, so take that how you will.