You need two pieces of J of you do not have F moulding. One to hold up the soffit and one to cover up the tops of the siding. If I have to do it this way I add a piece of Utility trim up side down inside the bottom piece of J, punch the top of the siding with a special punch that makes a rectanguler hole but leaves a tab so he siding can snap into place.
We now use wants called Vinyl cove moulding made by Certinteed for a far more finished look, and only use J around the windows, doors, and rakes.
It has a 1/2 track to hold up the soffit and the bottom part fits tight to the wall, Once the sidings cut to width we punch the top every 4' with a rectanguler special punch that leaves a tab stick out. That way the tab snaps into the cove and will never fall down again. It looks far better then just ugly J moulding.