"Dugie" wrote in message

Using screws in the new vinyl window may cause permanent damage, so I need
another method.

No more or less than in a wood window. All you need is two screws to be
sure the window is held in place to the sliding expander. In most cases,
once the window is closed, the screw holes will be below the sill and not
visible.

Only other way is to make a screw jack of some sort to put above the sash
frame and below the widow frame to exert pressure down on the sash. You can
use 1/2" threaded rod and a coupling nut in the center to expand them.