Pulse width of a stepper driver
If you increase your pulse frequency (and this is what you do when you decrease your HIGH time*), you increase speed.
And increasing speed means decreasing torque.
The internet is full of information on this. For instance, have a look here.

Vibrations also depend on speed - and of course on your system parameters that we don't know.
Generally, using microstepping can help to reduce vibrations. See also this discussion.