Just did this in a 5v5 game. Solo top Vayne with Cleanse/Flash. Garen with Ignite/Flash. At mid-late game I had to roam because our Graves fed so much and Garen was able to catch up in CS but we won nonetheless and by the time I left top lane I was already 7/1/5, 1 death being from a nice gank from Hecarim but even though I died I took both Heca and Garen down with me.
Just a quick tip if you will ever go Vayne top against Garen.. As I said before, always take extra wards. When you are fighting him and he backs but remains in lane, he will hide in bush as it is his best chance to fight you. Make him regret that decision by placing a ward and quickly using Condemn and using your Tumble right after your initial autoattack hit. Garen just got dominated on and as I said in an earlier post, the only way he ever touched me was when he flashed in, ignited, and spammed all his skills just to run back with 30% HP while I had 80% HP. Vayne is very effective against Garen because she has Tumble AND Condemn. You make Garen pay for ever setting foot in the bush. After he learns his mistake and never goes in the bush again, you just simply kite and harass like normal and you are already fed by the time mid game comes.
Garen had to come up witha sorry excuse like "You're lucky I took AP runes." Sure those runes would have helped but it wouldn't have changed much when he can't even touch me.
I ended this game with 18/1/7
My teammate at the beginning of the match were raging at me saying GL feeding. Surprisingly, I accumulated the least amount of death, highest amount of kills, and the highest amount of CS laning phase. The very people who accused me of trolling/future feeding actually fed their lanes and even their fed Ashe/Lux could not handle me as I could simply 2 shot them to death and their Heca/Leona would take maybe a few extra hits. Vayne vs Garen, look to snowball the lane but don't do it recklessly. Do it smart. This lane matchup is all about your reaction time and smart plays. First 2 levels Garen will be able to kill you easy if he flashes in, ignites and goes all in. If you are not good at reaction time, I suggest starting Condemn at level 1 and Tumble level 2. Don't be afraid to use Condemn when he's chasing you with his Q on and it looks like he will hit.
Also, never count out unorthodox picks nor underestimate your enemies because of such weird picks. It's about being smart and knowing how to counter. Choosing Vayne against Garen was an initiative under the motive of using Garen's strengths and weaknesses against him.