Whenever I open my gas cap, I can hear pressure equalizing. This has me concerned.

Today I took apart the EVAP components and tested the following: canister purge valve (attached to check valve between gas tank and canister, allows fuel vapors into canister), vent valve (attached to the canister, kills air flow into canister), MAP pressure switch (just above canister, I assume it reads fuel tank pressure), purge control valve (atop intake in engine bay, I assume it sends vapors from canister to intake). Everything seemed to check out fine.

I drove the car for about five minutes after buttoning everything up, and opened the gas cap afterwards. Again I heard the pressure equalizing, and it sounded like the air was entering the gas tank, making me believe that a vacuum is forming inside the tank when the vehicle is in operation. Any thoughts?

Perfectly normal. Without the gas cap in place, you would get a check engine light on and a code reading indicating a vacuum leak. That being said, there could also be a problem causing too much vacuum. I copied the following from Autointhe box.com: "Fuel pumps feed fuel to engine carburetors by pumping it from fuel tanks. Removing fuel causes a vacuum that, if not released or maintained at the correct pressure, becomes too great, causes pump failure and starves the carburetor of fuel. Until this vacuum is released, the car will not run.
Actions to Take to Cure the Problem
Most gas caps have valves fitted that automatically release the vacuum at optimum pressure. Removing the cap enables the car to run, but this solution should not be a long-term fix. If the cap has a faulty valve, replace the cap. Alternatively, some cars have a breather pipe fitted. If this pipe becomes blocked, clear or replace it. As a temporary fix, remove and immediately refit the cap to release the vacuum.
Conclusion
An older-type gas cap with no bodywork cover
Removing the cap prevents a tank vacuum, stops the car from stalling and stops pump malfunction. But it increases the risk of fuel vapor contamination, pump blockages, increased consumption, poorer performance and fire hazard. These risks are increased greatly if the cap has no bodywork cover.
Warning
Do not insert a rag in the open fuel tank filler pipe to prevent spillage and evaporation. This greatly increases fire danger."

Thanks. I did some further research and found that the EVAP lines may be clogged between the tank and the canister, or between the canister and vent valve. I'll check on this and report back.

When I first started troubleshooting this issue, the fuel system would hiss upon opening the gas cap, even after I only ran the car for a minute. I ordered a vacuum/pressure pump, opened up the FSM, and methodically tested every solenoid and valve that was a part of the EVAP system. They all checked out fine.

When I disconnected the line from the fuel tank at the vacuum cut valve, I definitely had hissing vapor blowing in my face, so I knew the gas tank was being pressurized (as opposed to my original belief that it had vacuum). Looking at the EVAP and fuel diagrams, I figured the obstruction had to be either in the vapor-to-canister line, or between the canister and vent. I took everything apart and sucked air through it with my tool. I even tested the fuel cut valve via the vapor line. Results were inconclusive.

Well, I didn't find a definite obstruction, but when I put everything back together, I no longer had any hissing or pressure issues. So, success. I believe the culprit may have been the water separator just downstream of the vent valve, as it was the last thing I disconnected last night, not bothering to put it back. This morning, when I began working on the car, I didn't really realize it at the time, but looking back on it, I don't believe that there was excess pressure inside the fuel tank when I removed the filler cap. My guess is that all the dust and dirt and road grime that has covered the underbody since before I bought the car also made its way into the separator and somehow prevented vapor from escaping through it.