You do not schedule regular Uber/Lyft vehicles. They are "on demand" ride services. You open an app and request a vehicle when you need one. There are so many you have no risk whatsoever of not having a vehicle available.
Minnie Vans are slightly different as they are a class of service available at Disney only bookable through Lyft. You still request them on demand, when you need it. There are a finite number of these specific vehicles and yes, they can run out. However, they are continually dropping off guests and I've found that usually, if the first request times out, ask for another and you'll probably get one. They have significantly increased the fleet since it began and it's getting easier and easier to get one
As to if it's better to wait for a bus, check the estimated wait times for a bus and check the app for an estimate on how long a wait on a car is and see what is longer to wait on.