Word floating around what's left of the "cypherpunk" and "Mixmaster" communities - Apparently, when using a VPN, something (involving math that's *WAY* beyond my understanding/capabilities, though the way the conversations I've seen went, it sounded at least plausible) happens during the handshake in setting up a TOR connection that can expose both ends of the VPN *AND* the TOR connection to "outside observers". Apparently it gets even easier if the traffic goes through a TOR node under the control of the "adversary", whoever that happens to be, since they can twiddle things in such a way that every connection made to that node will trigger the "something" that I don't understand to happen every time, rather than there just being a chance that it might happen during any random connection.
My understanding is that the folks running "honest" TOR nodes can, assuming they're paying attention, detect it fairly easily, and either knock the "naughty" node out of the TOR network, or route around it, but it's your basic game of "whack-a-mole" - A determined/decently funded adversary can toss up new ones that "look" honest faster than they're likely to be noticed and "blown off the network" by the "honest" ones.
E.T.A. - Had this queued up for hours, just now noticing that I never actually hit send.