Vacuum gauge are a must. Especially if you are doing infusion. I highly recommend the GDH200-14. This is a digital absolute pressure gauge. For infusion you can connect it to your resin inlet to get a reading of your part vacuum and leak rate.

Okay and thanks for the info...

If you are doing infusion or resin degassing, most infusion catch pots and degassing chambers have a gauge built in or attached to them so you won't often need another one. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

You don't need a vacuum gauge at all, exept for wet layup vacuum bagging. It can be handy, but it is usually not necessary. If you want to get one, the only thing you want to look for is how you install it in your setup. There are gauges with the thread on the back and others with the thread on the under side.
Or there are digital ones, but those are a completely different story.

Is there a specific Gauge I need for a vacuum pump?