an executable would normally cause an automatic vac ban. one that doesn't happen immediately would be based on reports and evidence. since VAC can't control all methods of cheating through their program alone, they can sometimes review evidence and make the bans manually. vac can't tell if it's an exe or a script, but they can make the ban either way. we know i2o4's been bhopping, we know he's used scripts, but we have no evidence that it happened on this server.
and yes, a player like i2o4 would have certainly been detected by us if he made a new account. new players playing well always raise a red flag. i personally haven't been in-game much recently, but when i am i always check into new names with good scores, especially in wcs.