The VNT-25, as used on some 89-90 crysler models, uses vacuum and pressure to control the vanes. (See picture) The keyboard in the picture should give you an idea on it's physical size.
My idea of control was to use the stock auto-tranny vacuum device on the IP to "control" the vanes at idle/low throttle for less exhaust restriction. Also to exercise the vanes and prevent carbon sticking of the vanes. (As many babied VW TDI's run into)
From what I have gathered on others that have used this turbo on g@ssers, running manifold boost to the pressure side of the actuator (With a vacuum check valve on gassers) will limit it to 9psi. A standard boost controller should bring this up where I want it.
HERE is a teardown photo gallery of a VNT-25.