If the cedar siding is in decent shape, not loose, and probably not rotting, I'd say leave it on. It is true that it is easier (cheaper) to trim out a vinyl siding job when the bevel lapped siding is removed, but I believe that it is worth the extra trouble to retain the exising moisture barrier and insulation of the cedar siding. On my 50 year old ranch the leading edge of the cedar siding came out flush with the brickmold trim around the doors and windows. The vinyl siding was to go over the blue board (accordian fold insulation ) which was applied over the siding to level the surface, and give a smooth surface to hang the siding against. Thst presented a problem with the insulation being exposed around the edges of the windows and doors, with the vinyl J-channel being applied over the insulation. To give an edge to apply the vinyl J-Channel against, I ripped 1" x 2" boards and applied them to the siding as a frame around the windows and doors. The siding contractor bent aluminum sheets to match the window color and covered the 1" x 2" 's. The J-Channel was then able to but against this frame. It turned out well, and I am glad that I retained the old Cedar siding.