I wouldn't write him off yet.
Nasri has only just became explosive this season and Vela has two years on him, so I wouldn't put it past the Mexican to develop in this department. He's also officially the 2nd fastest player in the squad.
He does fail to impose himself on the game, but that's why he's on loan. Getting some PL experience and game time under his belt will only do him good.
As far as I'm concerned, he has the raw ability and talent to make it here.