Android P is set to introduce Vulkan Graphics API version 1.1 support which will eventually, but not immediately, improve the overall visuals and performance of mobile games for end users. That said, this new version of the API is only a jump from 1.0 to 1.1 and there won't be a ton of changes that are leaps ahead in the graphical department, so even when the changes do start to come through as noticeable changes for end users, they won't be so different from before that users will think they're playing games on a newer generation system.
Vulkan 1.1 (which was released back in the beginning of March) will have some new features though that are user-facing, so all that will remain once Android P is released with support for the API, is for developers to start implementing those changes into their games so users will end up seeing them. That means that for the time being, Vulkan 1.1 will really only be hugely beneficial to developers, at least until they start pushing those changes to their offerings. As it currently stands Vulkan 1.1 doesn't have support within Android yet, but it's suggested that it might be added into the second developer preview once Google pushes that out, which could happen in the near future since the first version of the developer preview has been out for more than a month now.
If Vulkan 1.1 support is pushed out to devs with the second preview, then that will give developers time to play with it and check things out, and Google should eventually push it as part of the stable build of the next version of Android, though that build isn't likely to release before the end of the Summer. Vulkan 1.0 is already supported by Android and was introduced with Nougat, so there are already a fair number of games which are built on the base of Vulkan, though not all games utilize the Vulkan API for the visual components. The same instance is more than likely going to be the case once version 1.1 of the API is supported as well, as not all developers will build games that take advantage of the API.