I have noticed there are times it's far more appropriate for me to play her. If we are playing with all IRL people against AI-- then she seems to be the best choice. If I'm having to run a lane with a bot-- it can be a little slow. I've actually not tried her in pvp, yet-- but it would be interesting to do so.
Vayne is a real solid champ. She's an AD anti-carry/carry (attack damage carry) so generally you build attack speed, damage, and life steal. She's really quishy so if you play her you need to get used to her range, passive abilities (like her 'E' ability) and her positioning. She's seen a lot and can really wreck when fed gold.