However, they usually are designed around a target size. And it's quite possible that, scaled too small, much of the detail is lost. Hence why you'd still want different vector files for different resolutions.
Considering the topic of this post (apps), I would argue assets are usually designed around a physical size of the image; even when you have multiple target display sizes, it becomes a part of the requirements to design assets the way that looks decent on all phones. Moreover, I find this whole branch of discussion a bit funny because assets are already designed to be high res and later downscaled for target dimensions, so simply using vectors literally just pushes that task to phone instead of build server.
