Interesting question. I don’t think Vetinari would want to be on the Iron Throne – in the series, he’s a pretty consistent anti-monarchist, and I don’t think he’s quite suited to embodying what a king is supposed to embody.
But he’d be a hell of a Hand of the King. A genuine Machiavellian that Tywin only thinks he is, but with some sense of precision, subtlety, and proportionality, and a policy agenda (however gradual) of modernization.