I wrote THIS POST years back regarding adding vent pipe to a sanitary system of pipe and fixtures. I'm happy to write that it's simpler now than then.
Just use an endcap family (made to look more like a vent cap) the vent pipe at the top and your fixture count should propagate all the way to the building connection as expected. That of course assumes you don't have open connections somewhere else along the way. The example above shows vent pipe using a separate type and a view Filter based on Type Name.
Use the Show Connections feature to identify them if any exist. I won't be shocked if you find some.