Best way to affix downspout to vinyl sided house?
Honestly, I'd use nothing. The problem is if you ever remove the screw, then the hole needs to be filled. Mount the bracket high-up, tucked under a lap, so you don't buckle the siding while installing it. Check both ends of the siding for play first. Affixed in the middle with room on both sides, it should be fine. Use screws with a fully threaded shank; you're only going into plywood. They sell gasketed screws if you want to go nuts: