OK, I just did the first deal, and it does appear to be euros-only, but that still saves quite a bit over NOT taking advantage of the vouchers and buying elsewhere (e.g. Ilio) in dollars vs. euros.
I did notice that adding six 100-euro vouchers did NOT result in two free vouchers, so it looks like each purchase of four-for-three must be a separate transaction.
Also, if it wasn't clear from my previous post, the Ilio email makes it clearer that the two volumes of Historic Winds are separate purchases. I thought that likely, but due to how the information lines up on the webpage, it is slightly unclear. Just a formatting thing on the webpage.