Resurr You're mistaken. Vanadium does not have access to all your files. All apps including Vanadium are sandboxed.
If you want, you can enable Vanadium access to your 'Music and audio' or 'Photos and videos' by going to Settings > Apps > Vanadium > Permissions. But by default these permissions are disabled.
What you might be referring to is the file picker. This is part of the stock File manager app. When you want to open a file in Vanadium, it asks the File manager app to show you the file picker. In the file picker screen you can then select the file you want to open in Vanadium. Vanadium only has access to the file you select from the file picker, none of your other files.
Vanadium is not the only app to do this. Many apps do and it is the correct way for apps to ask for files without needing any file access permissions, since the File Manager serves the app for them.