What chemical is best to pour into plugged up plumbing vent?

Use a plumbers snake instead of chemicals.

It is clogged, use a snake or roto-rootor from the rental store. Chemicals are a potential hazard to you and anyone else that has to work on the clog.

Okay.

I have already put the garden hose with a nozzle into the vent with no success. The kitchen sink will not drain properly until the vent is opened. There are so many different kinds of chemicals for drains, so Im asking which one would be best to pour down the vent pipe?