How do these portent dice work, exactly? Do you just roll a couple of d20s at the start of each day and save them for later?
Pretty much. You can use one of the portent dice in place of any skill, attack, or save roll made by you or anyone you can see. No save/resist to have your roll replaced. You keep the dice until you use them or you have a long rest. At level (13? I think?) you get a third portent die to roll every day.
It's pretty frickin' sweet.
Crit portent dice are pretty awesome. But if you play a character for that, you're going to be disappointed, because portent dice don't make them any more common. The vast majority of the time you'll have like an 8 and a 13 to use, and you'll just be using them to guarantee a hit or a failed save. Which is very good, but not as cool as crits.
It's not about playing a character
for that reason, it's just good to clarify exactly what kinds of tools you have in your little bag of tricks. Especially when the rules get a little bit finicky or difficult to understand... And no, portent rolls aren't any more likely to be crits than any others, but it's the fact that you
know ahead of time what the roll is going to be that makes portent crits better than standard crits. Because if you do have a portent crit, you know exactly when and where that crit is going to take place.
Again, this was mainly just about clarifying whether or not the off-chance clutch beefwizard scenario was even theoretically possible, not about whether or not it was viable.