How do I add a glass break sensor to the Vista 20P?

It depends what kind of glass break sensor you are looking to install.

If it's a hardwired glass break detector, you can install it directly to one of the 8 hardwired zones on your Vista 20P control panel (assuming there is one available). If all hardwired zones on the control panel are taken, you can install the 4219 expander to add an extra 8 hardwired zones, at which point you could install the glass break detector without a problem.

If you're looking to install a wireless glass break detector, like the 5853, you must first get a wireless receiver, like the 5881ENH, 5881ENM, or 5881ENL. The receiver allows the control panel to receive wireless signals from the detector. You can achieve a similar result by using a 6160RF keypad/receiver, which has a fully-functional 5881ENH built-in.