Luke,
Here is a link to an older post. It is as stated, a valve that is closed when cold and open when hot. Set up as a part of the EGR system (I believe) when the car is warm and at a high enough RPM, fumes are pulled from the canister with the air intake. Fumes are collected from the gas tank "trapped" until the correct conidition allows the vacuum line to pull the out.
When I looked into mine, I didn't find a replacement (most said go to a junkyard), but just deciced to fix the one I had. Can't say it works perfectly, but it isn't giving me any troubles eiher. Heres a link that starts with my work on the TVV.
link
Chris