Can this product be used to clean vinyl siding over top (adjacent) of a metal roof.

TSP may actually be used on metal roofs; however, it is imperative that the TSP is diluted properly; about one part of TSP to eight or nine parts of water. I don't see any issues with cleaning vinyl adjacent to roof so long as the solution is not to strong and that the metal roof is rinsed thoroughly. Aluminum rivets, facia, roofing are more sensitive that steel.