Question about roof vent
The only things that should limit you are number of fixture units being vented (dfu), and the required slope of horizontal vent pipes (1/4&quot; per foot for 2&quot; pipe).
Drain Fixture Units
If too many fixtures are being vented by this vent, the size of the vent will have to be increased.
Since it sounds like you're venting a residential kitchen, there's likely only a sink. So you'll probably not have a problem with this.
Grade
Vent pipes must be graded and connected, so that they drain back to the drainage pipe by gravity.  Which means, any horizontal sections of the 2&quot; pipe will have to have a slope of 1/4&quot; per foot toward the fixture drain.
This could be a problem depending on the surrounding structure, since the pipe will be 6&quot; higher at the stack end.
1/4&quot; * 24' = 6&quot;