Yea... linesman blew that call, defender was on the line.
No he didn't, it's a clear offside. The rules are that there must be at least defending 2 players closer to the goal. Normally one of them is the goalkeeper, but in this case, the Mexican player was in front of the goalkeeper. It was a good call.
These vuvuzelas are seriously annoying, I might boycott watching all games by RSA hoping it's less annoying when they're not playing. I really see no point in them except to create this annoying buzz...