I am adding a bathroom to the second floor and wanted to know if I can tie into my vent pipe. However, it doesn't run straight up through the attic. It bends between floors and runs 9 ft horizontal, then runs up next to an exterior wall through the roof

Can I tie into the horizontal sections, to drain the new tub, shower and sink or is that going to prevent proper venting for my other plumbing on the first floor? Thanks

If you connect to the vent stack you will have to run vents on all the first floor fixtures that are using this vent. Would it be possible to run a drain line separate from the vent? You could then revent the new installation back to the old vent.
Let me know, Tom

Tom said it. You can't just run your second story drain into vent for first story fixtures. If it was on the same story, and pipes were sized right, you could, it would be called wet venting. Your best bet would be to get the second story drain down below first story fixuters and tie it in there(into at least a three inch pipe, as I assume you are adding a toilet). Then vent second story fixtures into existing vent up in the attic( or at least six inches above highes flood rim of second story fixture) hope this helps.

Thanks, I can run a separate line but was wondering if since I had that pipe right there, it would make life easier, but I will be able to run a new pipe. Thanks again

Good luck on your project, and come on back if you have any more questions.