What if i told you I had 2 levels on the Vayne?
So? I main support and marksman. If I'm caught out as Sona I'm screwed. If I try face-tanking a Riven I'm screwed. If I Crescendo an entire team we win the team fight. If I pew-pew-pew behind my allies I can pick Riven apart before she reaches me.
Consider the optimal play pattern for a champion. If you don't have any means to deal with getting caught out, don't get caught out.
2 levels isn't enough to beat Scissors with Paper.
Skill also plays a substantial part in LoL. Balance doesn't play a big enough roll to prevent any champion from hitting Platinum. If you die, 99% of the time it's because mistakes were made. Try to learn something from it.
What if i told you we met in the river?
You should usually keep to your side of the jungle. River is generally warded and dangerous to traverse, you'll often give away your position.
And there's nothing an assassin likes more than knowing you're alone and out of position, to far to flee to safety.
Also - River is generally warded, if you didin't see her comming because it's unwarded, you might want to ward it. It's a great place to keep track of enemy movements: and pretty important with Baron and Dragon being there.
Too Weak isnt an option. Bad Positioning isnt an option either because the only thing you can do against an ADC is run.
Supports are pretty much always to weak and always need to be in a good position. Support champions also tend to get the best KDA scores and win-rates by stat tracking sites. Janna left worlds season 4 with an 80% win rate, indisputably the best performance. Sona arguably had the best performance of any champion in the season 2 tournament.
LoL isn't about Street Fighter 1v1 tactics. It's a 5v5 game, and if anything: it's always favored champions strong in a 5v5 rather than 1v1.
Work on positioning and planning ahead. To weak is a reality many champions can overcome.
Also - the only thing you can do vs. an ADC isn't just running. Most are quite frankly terrible duelists, but extremely good at striking someone in the back and finishing off a fleeing opponent - be it from Lizard Buff slows, global executes, or just decent chase potential. Even if you'll loose to one, it's better to stand and fight: a Vayne that's allowed to shoot you in the back will take 0 damage in return. A Vayne at 50% can get nuked to 0 by a support's base damages, and certainly finished off by anyone else.
Not to mention the QWER rotation of just about any mage or assassin will often OHKO a marksman, making running and getting shot an inferior response to turning them into paste.