Large RPM and Vacuum drop when shifting from park to gear

You can do everything inside the distributor that hooking the vcan to manifold vacuum will do. You cannot do everything inside the distributor that hooking the vcan to ported will do. For a street car the vcan should be on the sparkport and you should build your curve with springs. @Mattax has good info about springs with long loops and specific curves that will bring in idle timing after initial.