That toilet is already vented by means of the main stack. There are limits on how many things can be vented on certain size stacks, but unless you've got some huge number of things going into that it should be fine. Ask the building inspectors or even better the building code. I've used a reference called Code Check sold at lots of bookstores or Amazon.com or even Home Depot that would tell you if you have enough venting for what you've got. I'm not a pro but didn't want to leave you hanging since nobody else answered your question.
Looks like the flange would go inside that hub but it looks a little high off the floor. Hard to tell from the photos.