Okay, so scrolling around the vault becomes very cumbersome when it reaches a 50% dig point. With the 3D engine trying to handle 2D rag doll assets, the routine aging processes, and the animations, it's just too much processor grabbing to keep the game smooth (we won't mention how much iOS 9.x has cremated the performance).

So what about a mini map with the ability to finger inside it to move around the vault?

Optional of course, but I think it would solve a massive problem of accidentally grabbing a dweller during normal scrolling. It would also offer a non-implied UI solution to navigation. Right now the UI has so many layers of what a click means, its interrupting game play. When raiders arrive and you're working on the bottom floor, it's a nightmare to try and "safely" scroll UP. You have to try and keep your fingers inside dead rooms or elevator shafts. If you're zoomed all the way in, forget it.

Adding feature to compensate a broken feature is called "adding a crutch". Better way is to fix the broken feature.

All I need - max zoom-out should have width equal to "vault width" + "tap zone". Tap zone is usually 40-48 dp (independent pixels). It should allow to scroll up / down through vault without dragging dwellers by mistake. Currently, max zoom-out has a free space, but it is a little bit less than recommended.

If a new feature for navigation is necessary, then how about joystick on the left part? Like ones, used in hack-n-slash RPGs and tank simulators (plenty of examples in Google Play)