Rock3tman here..
I bought a new VNT-15 actuator from Hans VW (Chinese made, but I didn't care as it was to be cut apart and modified anyway) for $49.00 - - much less than a new one from
Garrett/Honeywell.
The actuator was cut apart by grinding off the crimp on the perimeter, the new spring was put in on the other side, the vacuum nipple was cutoff and replaced with short
6mm threaded rod section welded/brazed onto actuator, a small boost port tube was installed with epoxy on the actuator to input the post-turbo boost pressure into the actuator and, finally, the actuator was refastened on the perimeter with 8 very small bolts, washers, and locknuts. Whew!
To help you with spring selection: from 0 - 20 psi, the actuator changes in length by about 12 - 15mm. My adjustable linkage translates that to full motion at the VNT-15 vane arm from fully closed to fully open. To calculate spring strength, take area of membrane in actuator and multiply by the boost pressure you intend to use, over the distance from 0 to 12-15 mm.
Hope this helps, Rock3tman