To properly re flash a chimney with vinyl siding you must remove everything around the chimney including the siding and any old metal flashing that might be installed.
Vinyl siding chimney flashing.
Leave a 1 4 inch gap between the step flashing and wood siding or cement board.
Inspect the sidewall around and underneath the fascia and inside the house for signs of moisture damage and repair any rotted areas.
Nail j channel for vinyl or metal siding onto the flashing.
Roof chimney leak repair.
Install the kickout flashing underneath the first shingle photo 1.
On a vinyl job moisture gains access to the back side of the vinyl at all sorts of points.
Add felt if needed before nailing the new wood on and prime the new wood on all four sides before installing it.
Then an ice water shield underlayment must be installed around the chimney.
Gable ends are their own can of worms as reguards uphill chimney flashings.