So I changed the folder name back to vault, so I could do it the right way after I fix it.

md
# dendron.code-workspace
{
  "folders": [
    {
      "path": "vault"
    }
  ],
  ...
}

md
# dendron.yml
workspace:
    vaults:
        -fsPath: vault

I opened and closed vscode and tried the

Developer: Reload Window

but received the same empty json error both times.