I am currently getting bids to replace my old aluminum windows with vinyl replacement inserts. My house has vinyl siding over a 3/4" foam backer board underneath. Vinyl J channel surrounds the bulk of the windows rather than wood and I have had different methods of installation proposed to deal with this. The first is a window insert with a nail fin, second is a window with nail fin and the immediate surrounding siding removed to install flashing with the siding then re-installed (this is 12 year old siding that be re-installed) and finally, installation with a nail fin and low pressure foam with no flashing. Installation being critical to a good outcome, what would people recommend in this situation?