WHERE the vent is in relation to the trap and its connection to the plumbing system determines which fittings will be acceptable. IF the vent were in the horizontal trap arm, BEFORE it connected to the vertical pipe, then you could use any fitting you wanted t to connect it. But in your picture the vent is AT the connection, therefore only a sanitary tee, or back to back fixture fitting for two sinks, would be acceptable. A Y-1/8 bend is usually a single fitting called a "combination" or combo for short. An 1/8 bend with a Y, ( or a Y and separate 1/8 bend), does the same thing, but gives a degree of flexibility while taking up more "space".