None of the fares mentioned here seems like much of a bargain, and to answer a specific question, the ValPak codes are no bargain to Europe either.
Forget using these certs thru the end of October. NWA no longer includes any of the tax/airport/security charges in the prices printed on the cert. When these are included, Europe from Zone A is about $710, from Zone B $760, from Zone C, 810! As my flatmate during my London days would have put it, Bollocks.
The ValPak fares drop $200 from each zone from Nov. 1 - Jan. 31, so is that a bargain? FTer bublik mentioned getting a $424 fare Memphis-London Nov.24-30, so I ran the numbers for that booking with a PD935. From Zone A Memphis, the fare is now $508 with the PD, $517 without. From Zone B Minneapolis, NWA has its own "special fare" code squatting in the fare designator box at $551 and nothing changes when you enter the PD. Ditto from Zone C San Francisco, where the "special" is $541 and nwa.com ignores the PD.
I changed the dates to Jan. 24-30 and got higher fares on searches without the cert. MEM is $547 (cert fixed at $508). MSP is $581, and this time the PD is accepted at $548. SFO is $571, and the website still refuses to give a PD price.
By the way, all these searches were to LON and all got Gatwick, the NWA destination. KLM and the more convenient Heathrow are out of the picture. The PD makes no difference or very little; its fixed prices might save more on the more obscure European routes, but they're still expensive for the time of year.
The bottom line, I think, is that these are just sticker prices and buying European tix now, well ahead of the trip, is about as about as smart as agreeing to the first price quoted by a car salesman! When reality sets in (empty seats and competition to fill them) we'll see the usual fares around $400 by mid-October, and not much over $300 for non-holidays in late fall and the winter.