Nothing you add in Visual Studio is going to “stick.” Unity will always wipe out the solution and project files and regenerate new ones according to what is in the Unity side of your project.
You need to do 100% of your adding projects and references withiin Unity, then use Assets → Open C# Project to see it in Visual Studio.
Remember: you do NOT need any IDE or compiler to use Unity. It is NOT part of the development toolchain. Unity is the full toolchain. You could simply edit all scripts with Notepad or even vi if you want.
This may help you with intellisense and possibly other Visual Studio integration problems:
Sometimes the fix is as simple as doing Assets → Open C# Project from Unity. Other times it requires more.
Also, try update the VSCode package inside of Unity: Window → Package Manager → Search for Visual Studio Code Editor → Press the Update button
Also, this: No suggestions in Vscode