And no one's gonna find out eventually? Unlikely. Any admin worth his money will see something suspicious when the stuff passes through the routers.
Note: I'm not talking about how to detect this on your own machine. I mean detecting it *in general* so that information comes out that this stuff even exists. For example, lots of people are infected and part of a botnet. But we *know* that botnets exist. The people infected might not know, but it's common knowledge that botnets exist. If hardware backdoors appear, it's a matter of time until their existence too becomes public knowledge.
If there were a backdoor in vpro then it would likely only be used for targeted attacks, not random mass monitoring. So unless you were targeted in such an attack, and logged all traffic coming out of the PC, then no, you would never see it. And (like Stuxnet) the "backdoor" wouldn't be called a backdoor, it would officially be a "bug" that the NSA happened to find and exploit. Zero power remote bugs were created about 70 years ago, I'm sure things have advanced since then, so no, it is not impossible. A buffer overflow in vPro microcode could enable remote upload of code, which could covertly communicate over the WiFi chip on an unassigned channel, or frequency hopping.
Remember that Stuxnet had network command and control, and spread across local LANs of secured networks, and yet these crackshot admins didn't notice the odd network traffic at all, for years, until the authors released a version that accidentally spread across the open internet. It is entirely possible that they have even better exploits that they reserve for non-worm local only usage.