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.

I know of two reasons for vibrations when driving stepper motors:

• The steps time intervals are not regular causing speeding up and slowing down during the rotation. You want to make sure there is as little jitter as possible in the pulses. If you are using a computer (e.g. PC, RPi) you can't achieve this in software. Without an RTOS you can't get guaranteed timing in the execution of the code. If you are using a microcontroller to generate pulses (e.g. Arduino) you will get a lot less jitter. If you want to get even better you can disable interrupts (assuming you don't need them for UART communication) or use a timer interrupt to generate the pulses.

• At low speed, half steps can cause vibrations. By reducing the steps to 1/8 or 1/16 you'll get smoother movements. Of course you'll have to pulse your controller x4 or x8 more/faster for the same rotation. And you'll get more resolution.