How do you clean a sewer vent pipe?

No. You vent air from a sewer line.

It must be plugged up. try to snake it out.

you can try a garden hose down the vent on the roof use it like a drain snake with the water on

Very rarely will a roof sewer vent need to be cleaned. On the occasion that you need to remove debris or a nest, the best way is to try a shop vac. Then you could try a hose to clear any other items...

yes.