Launcher build ships with VulkanRHI Android binaries but do not seem to support Vulkan on Android from the launcher build without source.
1. Install 4.14 preview 2 from the launcher, exclude source code.
2. Create a new project
3. Observe that Project Settings, Android, Support Vulkan is disabled and the tooltip says it requires Github source
4. Manually set bSupportsVulkan=True and bBuildForES2=False in the project's DefaultEngine.ini, restart editor
5. Launch on Nexus 6P
Crash "Assertion failed: bBuildForES2" indicating VulkanRHI was not linked.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-38304 in the post.