For me, whichever of the Merc drivers is WDC will have the "honor" of having a title more rightly attributed to the car than to driver brilliance. The overwhelming superiority of the Merc this season is far greater than anything I have seen since Mansell's WDC. That one, IMHO, will always be tainted because all of the driver "aids" (eg traction control, stability control, ABS etc) took the driver out of the equation in a manner never seen before or since (thank goodness).
I don't see how Vettel having an off year should raise questions about his driving ability or the deservedness of his WDCs. I can't think of a single WDC that didn't have the occasional "off year".