Definitely agree that Tristana is underpowered (Chaox was the only good Tristana I ever saw... and he was leagues ahead of the next guy).
Not sure I agree that Vlad is underpowered. He has really good harass and quite a decent burst. His ulti and pool also have high utility in teamfights or lane.
i disagree that tristina is underpowered, when played right can lead a team to victory.
I agree that any champion when played right can be good, but also some champions are easyer to play. That said its very hard to determine who is underpowered. But slightly easyer to see those who are overpowered.