Could one crack?
Heat treat has gotten much better over the years though.
I use an X-fixture that doubles the force from the vice, while also providing a hard stop.
You do the job very slowly.
Measure the slide with a 0.0001 micrometer at a couple of points ( I place marks with a white pencil).
I usually set the stop at less than the total movement I need and clamp the slide up, then wait about 20-30 minutes.
Release slide and measure.
Adjust stop and/or time until desired dimension is achieved.
It is not just a muscle it till it moves thing since the slide also springs back when released.
It might take anywhere from 4 to a dozen 'squeezes' to get things as desired.