Actually, the RoleProvider implements a singleton pattern and does not have a public constructor.  As such, you are correct in accessing the object through it's Instance() static accessor.  You won't be able to instantiate the class directly.

However, I beleive your approach might be a bit flawed when you use the RoleProvider in the first place.  The RoleController exposes a GetRoleByName method that would appear to better meet your needs:

Dim RoleController as new RoleController()

RoleInfo info = RoleController.GetRoleByName(portalId, roleName)

If you haven't already, I highly suggest browsing this object with Visual Studio's Object Browser to get a better feel for the class's topology.

Brandon