you will always kill a squishy faster than a tank

The ADC's priority in teamfights is to destroy the enemy frontline. Tanks make up that frontline. It's your assasin/mage/bruiser's job to go on their backline (damage dealers).

I would say the ADC's role is to shoot whatever is close to you if thats a tanky mundo then yes if it is a squishy support than yes. If there are two or more potential targets than the ADC should pick the one with the highest priority. Focus is more important for bruisers and mages who to maximize their effectiveness need to put their dammage on the right target.

If Mundo is closest to Vayne and that's the only target she can safely attack,...Yeah. Focusing Mundo is the way to go for Vayne.