I'd cut the copper about three inches before the stack and inspect it. If it looks like it's in good shape use a no hub clamp. They make special ones for copper to PVC connections, don't get a regular 2in no hub clamp as it won't fit.
Failing that... I'm assuming since the stack is copper that your two inch is soldered into the stack? If that's the case I'd cut it a few inches short of the stack, unsolder it, and clean out the Y very well. Then I'd do one of two things. One option is to solder in a new short piece of copper. The downfall here is they'll make you buy 10ft of 2in DWV copper for probably around $100 and you'll only use six inches of it. Your other option is if you can find some sort of street male adapter that you can solder into the Y on the stack. Then use a female PVC adapter on that. For soldering two inch you'll need a MAPP torch, propane won't cut it. You might need an acytlyne (I never could spell that word) torch to unsolder that fitting depending on how much crud is on the inside of it, what kind of solder they used, etc.
As for the vents you should be able to cut them and use a copper to PVC no hub clamp. The vents should be strapped to the framing and able to support themselves, if not the PVC is probably more rigid then DWV copper, especially worn DWV copper.