I hear allot about Brave and Duck Duck go, but it sounds like Graphene endorses the Vandium Browser as the most secure.
Can I simply use Vandium as my browser in all profiles as my only browser? Do I download Vandium from F Droid or Aurora?
Yes, Vanadium is the most secure and it's recommended to use it. It comes with GrapheneOS, don't need to install anything. If you add another profile, Vanadium will already be there, again no need to install anything.