Only use vet B.
I'm running the pipes for a second floor bathroom in a new addition. I have the lav and shower draining to an exterior gray water area and the water closet with it's own 3" pipe going to the existing septic tank. I was hoping to get away with only one vent for the lav/shower and have attached a picture of the layout. I wondered if I could go with just vent A or just B or if I needed both. Any other comments are welcome.
"A" is redundant, useless, and strictly cosmetic.In fact, there is no condition where "A" would be a proper vent.