Install door on sloped deck

Deck is attached to ledger with joists slightly sloped down away from house as was covered with plywood and waterproofing vinyl. Half of deck is enclosed as a sun room with a sliding glass door on joist line leading to the same deck surface. Existing install was to place door directly on vinyl and shim the far end and fill cavity with silicone. There is flashing on outside but leaking. I would like to remove door and install a sub sill and then a Suresill device and then door. However, there is a slope where sill would go. How can I best level that sill while keeping water out. If it was a level surface I would use a piece of Pvc board with lots of caulking in door opening then sure sill etc, but the slope is worrying me.