Huh?
Of course he didn't have evidence of ball tampering, he saw unusually early reverse swing and went to the match referee to see what was up with that. There are a number a possible reasons, hence the asking.
"Hey there Mr Match Referee, I noticed that the ball was reverse swing really early. What's up with that?"
"Well Daniel, we saw nothing untoward, so I guess it's just got to do with the balls they use here in England." (or something like that)
"Oh thanks Mr Match Referee. I just thought it was worth it to see."
And he was happy.
Just out of interest, just say that a team was doing something untoward, would the process be any different?
Answering such question is part of a match referees job, just like a supervisor.
Or is a captain suppose to just see something strange to him and go "Oh, thats strange. I don't know what's happening, but I better not ask any questions or people will say I am accusing the opposition of ball tampering."
Something happens at work that you have never seen before, what do you do?
Leave it when it could possible be something of importance? Or go up to your supervisor and say, "Hey, it's probably nothing, but I saw this happen. Has that ever happened before? Do you know what it is?"
"Oh, that has happened before. It is nothing major, but thanks for coming forward, I will get the technicians to have a look at it."
If Vettori went to the Match Referee and said "I think Pakistan was ball tampering" without evidence, then that's an accusation. But as far as I can see, he didn't do that. He had no evidence of why the ball was reverse swinging so early, as he was busy playing, so he asked the Match Referee. Is he suppose to ask around everyone to ask as to why the ball did so, is he suppose to look through video to see if something happened to the ball during the match? Oh, that's right, there is this guy whos job it is to watch the match, lets ask him.