Am installing a 1/2 bath upstairs, directly over the main bathroom. The current vent stack runs right up thru this room, can I plumb the new 1/2 bath toilet directly into this vent? If so, I assume it should connect to the main sewer line downstream from the main downstairs toilet? Will a 3" waste pipe suffice (I would like to bury it in a 2X4 wall)
You can tie your vent pipe(s) into that stack only, and as you suggested, the 3" waste line will have to run down to the basement/crawlspace and tie in below the first floor fixtures. You can fit a 3" Pipe in a 2x4 wall but only if there are no fittings on the pipe; any fittings will make the stack wider than the wall cavity and force the sheetrock to bow outward.
Your vent(s) can be 1 1/2" (if each fixture is vented individually), or 2" (if it's a single "wet vent" for both fixtures.