We've been toying with doing something like this. The original intent of vouching was to rescue good posts that happen to be [dead] (e.g. because a banned user posted them) which is why we didn't implement it that way at the time.
I think that this is a great idea. I imagine that it's very difficult to differentiate between users flagging things that really don't belong on the site and flagging things that they dislike for more personal reasons. Showing a "vouch" link to high-karma users on stories that are being flagged would make it easier to separate the two cases.
