You generally can't refill scrips until you're down to a 2-3 day supply**. I switched Part D providers this year, and figuring that they didn't know when I had last refilled my V scrip I tried to get it filled 4 days early because I was going to be picking up two other scrips at CVS - when I got to the register, no V - they said "sorry, insurance rejected it, you're one day too early". Electronic record keeping, I suspect related to the opiod crisis, means every pharmacy, and apparently every insurer, knows exactly what you take and when you last had a scrip for a given drug filled.
** there has to be a work around for going on out of country travel, but don't ask me how that works, especially if it's a DEA scheduled controlled substance.