What's under your vinyl, foam or something not structural I presume. You could remove a few pieces of the vinyl, very easy to do, cut out a couple of squares of your sheathing between two studs, and replace it with the same thickness plywood. Fasten it good with screws or nails, caulk all the way around and then install a light mount block made for vinyl siding. You can get them at Home depot. They are plastic and have a trim ring that pops off and goes over your siding and looks like a piece of channel all the way around. You have to cut your siding around the block, leaving about a 1/4" of clearance all the way around. Here's a link for them: http://www.dinesolbp.com/mounting_blocks_Standard.php
I am typing this on my deck as I sit her and listen to my bose speakers mounted on what else, light blocks. Nice evening....hurry and get yours up!