To add a prebuilt file to a build, find a .mk file which makes sense for your project and add the file under PRODUCT_COPY_FILE. It looks like this
PRODUCT_COPY_FILE += your/build/tree/$FILE:path/to/location/on/device/$FILE
Using a separate vendor tree is following good domain-driven principles...but is perhaps overkill for a single binary. Examine the makefiles you already use, there is often prebuilt apks already being added that you can piggyback on. device
and existing vendor
trees are good places to start looking. Maintaining an entirely new project is not something to be taken lightly, in my opinion.