I think the complete opposite, tbh.
If Vettel has a good race without mistakes he's usually in the top 3 at the end but he has already done several mistakes this season. Without these mistakes, he could even be right up there with Button on the points table.
Webber on the other hand is very consistent and finishes most of the time. At least, I can't remember a big mistake from him this year yet which caused him to DNF.
Therefore, I think it's not that ridiculous to think that Webber is more likely to be WDC than Vettel.