I'm looking for a cheap vacuum sensor. So far my research has drawn a blank, so I defer to you guys. It doesn't need to be very sensitive, it just needs to be able to tell the difference between two states. Basically how it needs to work is this; I'll have a vacuum pump hooked up to a small hose, which is normally unobstructed. I need something that will recognize the pressure spike that comes with an obstruction. Thanks!
How about a plastic big with a switch - when there's extra vacuum available due to the hose being obstructed, the bag will compress and close the switch.
#define cheap ?
CrossRoads:
How about a plastic big with a switch - when there's extra vacuum available due to the hose being obstructed, the bag will compress and close the switch.
Not a bad idea. However another thing I need is reliability, which is why I'd prefer not to make it myself. And by cheap, I mean as cheap as possible 
Your local Auto Supply store.
However another thing I need is reliability, which is why I'd prefer not to make it myself. And by cheap, I mean as cheap as possible
Usually, cost is proportional to 1/reliability.
A common type is called a "MAP sensor", and appear to be obtainable for less than $5.
These actually measure absolute pressure, but any pressure less than atmospheric pressure, you can interpret as a partial "vacuum".
What sort of " vacuum" do you actually expect to be creating ?
Any of the barometric sensors used to estimate altitude ( such as BMP180 ), will accurately measure pressures down to about 1/4 of an atmosphere, which is a lower pressure than your vacuum cleaner is likely to create.
Both good suggestions. I'd tend to favor the BMP180 because it's already commonly used with an arduino. But I guess the real answer to your question is I don't know what kind of vacuum to expect. Realistically it only needs to notice the change in pressure though, so the BMP180 should work nicely. That is, provided there's a way to get it to sense pressure inside a hose.