Please clarify. Are you talking about MS Visual Studio? Or VS Code (aka Visual Studio Code)? And are you wanting to connect to access data from the database, or to edit classes/routines within the Cache database?

As other have noted, it's the IRISLIB database, not the IRISSYS one, that you need to re-mount writable.

Today we released a new version of this extension. The connection mechanism is now simpler. Details are in the README, but here's a recording of it in action: