how to install mounting block on existing vinyl siding

My house is a 50's cape which originally had clapboard siding. The previous owner put a layer of rigid foam over the clapboard, then vinyl siding. As far as I can tell, from looking at the wall stack-up from the bottom I see:

1) 3/4 tongue and groove sheathing
2) An unidentified, ~3/4 wood material
3) Clapboard
4) Rigid foam
5) Tripple 3" Vinyl

Besides the foam I couldn't really tell you where, if anywhere, there is some sort of moister barrier.

Now I'd like to install a new exterior light. Which means that I need a box, and some sort of mounting block. I know that normally I would begin be installing the box recessed into the sheathing, then installing a two piece vinyl mounting block. Something like this image from hammerzone.com (minus the electrical box):

However it seems like this method isn't going to work in my case. Additionally it is my understanding that you cannot assume there is no moister behind your vinyl, that's why there is normally a moisture barrier. So in that case I really don't understand how this method would prevent moisture, which may have already gotten behind the vinyl, from getting into the electrical box. I realize that I could cut the box in, then use a mounting block designed for retrofitting, like this one: