I have this nagging feeling that I *must* be missing something, as Elyv and WW are both significantly better players than I am (and WW is more skilled with the simulator to boot), but...
I tried simulating WV/Peddler/Mountebank and WV/Peddler/Witch, and they both lose to their respective BM+2X strategies. Tweaking the buy rules gets me close, something like 49-46, but still on the losing side (and, perhaps I'm interpreting this incorrectly, but it seems like if buy rules make a big difference that makes the strategy harder to play correctly).
I did find that WV/Peddler/Chapel and WV/Peddler/Steward both thrash BM pretty soundly, but that's not too surprising since BM doesn't really benefit much from trashing. WV/Peddler/Warehouse and Cellar seem to be pretty good too.
So I think I would feel comfortable saying that this is a reasonable combo on a board with good trashing or good sifting (I bet it's pretty good with Spice Merchant, Stables, and Cartographer), but it's not that great otherwise. That is, in the context of the original question, Embassy/FG and Governor/Militia you can pretty much just buy a bunch of those cards and ignore the rest of the board, and you'll likely be at least competitive (my definition of a combo). Not so with WV/Peddler.
Again, I have a nagging feeling that I'm missing something. Please tell me what it is!