I think you are discussing something completely different than the people you are disagreeing with. Leading a team at a GT, 'riding for GC' and especially winning a stage, as yaco mentions, doesn't necessarily require being a strong candidate for the overall win. Cycling sure was more fun when more teams had legitimate GC contenders, even if many of those quite rightly weren't expected to be able to win one. The sport would be a much better watch from having riders like Vine leading smaller teams, in my opinion.
As for Vine's chances, I'd put him quite far down the list of people likely to win a GT if they were leading a team, but I also think that worse riders have won GTs. At his absolute best I think he has the level needed for a random outsider GT win. And I don't think his percieved lack of technical ability will be the main hindrance, although a sterrato stage wouldn't be ideal of course. Wiggins won a GT. Menchov won multiple. Zakarin podiumed one.