It is more likely a bad toilet seal or a cracked sewer line than the vent pipe. Your sink and tub have or should have p-traps so even if the vent was blocked sewer gas would be blocked by the water in the trap. If your vent was blocked it would cause drainage problems.
Check your drains.
I have to clean the drain pipe in my shower because the walls of the pipe gets a ring of hair, grass, or whatever will stick to the pipe at the trap.
A quick swipe with a pipe cleaner looking brush that Sandy found, and a spritz of bathroom cleaner, makes our problem go away.
Though this works for me, your mileage may vary.
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If your bath has a floor drain, it may have evaporated the liquid seal out of the trap; it can also happen to a seldom used shower or tub drain. To remedy the problem just pour a qrart of water in the drain the first day of each month.
If that is not the case, see if you can open a sewer clean-out outside, drop a small smoke bomb (from a plumbing supply store, its just a small thing like a cherry bomb, except it doesn't blow up) in and blow the smoke upstream with a vacum cleaner with the hose on the discharge side. Someone needs to be in the house to see where the smoke leaks out at the same time. Some use peppermint liquid in this test but it is very expensive and is still invisible.
My bathroom started to smell bad a week or so after my new tub was installed. They messed up the overflow seal and there was water standing in the wall. There is the bowl like thing cut out in my slab where the tub pipes go and it was just full of water.