Toilet needs his own vent pipe, you see them on the roofs coming out, those are the vents  for drain pipes.

Install one for your toilet.

What you call sewer vent cap, is actually where you connect the drain vent pipe up true the roof.

Since it is under the sink it is also vent for the sink.

We call that incomplete installation.

If you do not like drilling holes in your roof, you can have it exit true the wall.