My 1940 bathroom plumbing was just replaced and I am questioning whether the set-up is proper. Here is a picture. Long 2” pipe at bottom is for the bathtub/shower, fixture at top left is the toilet and then sink at the pipe on the wall. Sink connects into the main vent stack which the kitchen fixtures and an upstairs bathroom connect to as well. The main vent stack starts in this bathroom at the right of the bathtub drain. So sink has to drain into the vertical vent stack and around past the bathtub connect, down to the bottom 3” horizontal pipe then into the main drain stack that the 3” connects into. The sink is the only fixture in the house that drains into that vent stack.
That's fine, at least under the codes I work with, since you havn't specified your applicable code. Probably fine, period.
The vent stack is a dry vent above where the sink connects, meaning nothing drains into it. Below that point it's a "bathroom group wet vent" and correctly picks up all the fixtures, with the toilet being last in line. This is explicitly allowed in code.
For example, Section 912 of International Plumbing Code.