I think the more measured way of seeing it would be to recognize that Walcott will take time to fully recover and get up to speed; Vela is a player who roughly offers what Walcott does on the right wing. The added bonus is that he can play through the middle as well, especially if we are chasing a game and want another striker on the pitch.
Vela would be an option, but I very much doubt that even Wenger thinks that Vela will make up for the 30 odd goals we scored less than the top 2 this season.