I've started on a similar project, but I'm in my busy season, so it will wait until summer. There are a couple of approaches you can take.
1. The simplest is likely using a pressure switch from joe woodworker and connecting to an input. You don't want to hit an e-stop as that can result in lost steps and the need to reset your axis locations. A few relatively simple lines of code that calmly stop the machines motion and display a warning is all that is necessary. The part would be:
http://www.veneersupplies.com/produc...er-18-NPT.html
2. A programmable system that gives greater monitoring of other components in your system might include a plc or an adruino utilizing a barometric pressure sensor. This requires more programming and some elctronic experience. but can be used to perform a similar task of machine shut down based on vac pressure or any other sensors you might wish to implement.