Just adding to what
@SeriousBug answered:
There are a lot of people requesting floating windows for VSCode (last time I checked it was the number one voted feature request in the repository:
https://github.com/Microsoft/vscode/issues/10121)
There are workarounds suggested by other users in the thread to open the file in a new instance of VSCode. This sort of works, but will not be useful for local graphs because Dendron relies on seeing what note is open in the current instance of VSCode.
If you only want to see the full graph, you can follow that workaround, but it will not automatically update in the new instance.