I think that's the clean-out. You can't use that. But yes, the main DWV stack is vented. What you need to do is connect to the main stack by fitting in a Y fitting, which is pretty much just like the clean-out. If it's cast iron it won't be easy, but if it's plastic or copper won't be difficult at all.
[COLOR="DarkRed"][B]~ ToolGuy ~[/B][/COLOR]
[I]Save money on tools at [URL="http://mytoolbox.net"]MyToolbox.net[/URL]