If the Vice President has the authority you say, what prevents Harris from tossing the electoral vote in all the states Trump wins?

That's a strawman. We were talking about disputed electoral college votes and asking the respective states for confirmation of the votes being for the one candidate or the other. That is a completely different thing to unilaterally changing the wrong vote to the right vote.

If Congress has no role in the vote count, then it would say "The VP counts the votes by himself in his office and then announces the winner."

But Congress does have a role: to bear witness to the counting, to give confidence to the public that the declared result was true and accurate.