One vial with a valid prescription number & doctor information "usually" isn't going to draw any flags. Not saying that you won't get some dimwit in customs that will make life tough for you, but they will be the types going through your aspirin too. What most of these guys (customs) are on the lookout for larger items that would indicate that someone is moving larger quantities of drugs that obviously amounts to illegal trade and the exchange of currency.
Versus, if you had 10 vials or beyond, it would raise flags ... With anything, never say never, but I wouldn't worry about 1 vial with supporting credentials. And yes, I've been to several countries, it's never even been addressed, even when they've gone through my entire bag tooth and nail.
