Thank you! All signs point to it being a clogged solenoid. It could be the timing (since it first appeared immediately after a valve job) but the car is running so well that hopefully it is just a clogged solenoid.
Today I'm searching around for the 32mm (1.25") deep socket. Autozone loaned me a 32mm axel nut socket but it had a change of width half way down that wouldn't let it go further down the Vanos exhaust solenoid. Next I'll try to find the thin-walled shower valve socket/wrench that others mention as a way to get the solenoid off.