What is a Sqlite database doing in my project's .vs folder?
This file seems to be related to the "Lightweight Solution Load" (LSL) functionality in VS2017. If you disable LSL in your solution, delete the file from the directory and open your solution you will see that it is not recreated. However, enable LSL for the solution again and open your solution, you will see the file created again. I presume it is being used as a form of cacheing information to enable the LSL functionality.
As an aside, all info in the .vs folder is supposed to be safe to exclude from source control.
If you ended up here because you are getting
Git failed with a fatal error. error: open(".vs/Onion/v15/Server/sqlite3/db.lock"): Permission denied fatal: Unable to process path .vs/Onion/v15/Server/sqlite3/db.lockThen Team Explorer > Settings > Repository Settings > Add /.gitignore. It should now have .vs
# Visual Studio 2015 cache/options directory.vs/Fixes issue.