I would use a water hose - not connected to water - just get on the roof and run it down the vent. If that is not obstructed - go to the next step.
However, that sounds like the drain part of your sewer line is partially blocked. Plunger it pretty good, if that doesn't work use a snake, if that doesn't work - find your clean-out T in your yard and check that out.
Call a rooter company in the A.M. A clogged vent pipe wont cause such a dramatic result, even if someone pounded a tater in there. It's worth the hundred bucks.