what is the smallest diameter pipe that cab be used for a vent?

I'm roughing in a bathroom in a basement. The walls have 2" studs on them and they are pretty much butted up against the wall. I'm using standard 2" drains for the shower and vanity and 4" for the toilet. I am able to get a 2" pipe for the toilet vent. Can I use 1 1/2 pipe for the vent for the shower and vanity??

1.5" is sufficient for your application.