It's hard to accidentally vouch a comment, just like it's hard to accidentally flag a comment (submitted articles are another matter, I have accidentally flagged a lot of submissions since that's an option on the main page itself). You have to first select the specific comment (reply or click on its time stamp). Then you have to press vouch (or flag). You cannot vouch for them directly from the general comments page.

So I'd wager close to 0.