Both of my Foxes have broken MVAC vacuum reservoirs (the one that resides in the passenger fender). Is the emissions reservoir (by the coolant reservoir for 3.8 cars) the same? I'd be easier to pull one of those at a JY instead of going into the wheel-well.
On my 87/88 near the radiator is the gas vapor canister? If so, its a different part.
Here is a picture of the one under the fender. Its the big round ball. ... just a canister with one vacuum hose going to it.
(http://mysite.verizon.net/junkyardjoe/vac_resv.jpg)
As long as you can connect the hose and it holds vaccum about anything would be fine. Some of the older cars used a tin can with a nipple soldered to it.
OK, definitely different than the emissions reservoir.