I need to replace the siding (parged stucco) on the chimney. I want to make sure that the cap is also 'done right' so it ties into the siding (whatever product I choose - either vinyl or vertical wood). Where should I start and what should I look for from a contractor/siding company to ensure the cap (top) is done properly, so that it is flashed around the top and base properly, and so that it ties into the siding the right way?

The most important issue with respect to the area where the siding meets the cap is not so much the flashing, but a proper cap. One of the the links below describes the features of a proper chimney cap. Otherwise, the siding should terminate at the underside of the cap the same way that the cladding (siding) on a wall terminates at a soffit. Vinyl or metal cladding will have a "J" molding, while wood siding usually uses a piece of wood trim up against the cap/soffit. The capillary break on the cap should stop water from running back behind  the top of the cladding.

At the bottom of the chimney, the base flashing will be no different than any other chimney. Again, one of the links below describes this area. Since this chimney has siding, though, you won't be using a counter-flashing. Instead, the cladding itself will act as a counter-flashing, just as if the roof were meeting a wall instead of a chimney. Make sure that the cladding does not come right down to the roof surface, especially if you use wood, which will rot. There should be a gap of about 2".