By the way, in the USA ATC's will separate IFR aircraft when in radar covered class G. It's only in Australia that ATC's are not allowed to do this and are specifically trained just to give a traffic information service.
(US) - In this instance it depends on the controller providing service. Approach controllers are required to provide separation services for IFR traffic from known traffic - IFR or VFR. ref 3-1-1.
However:
5-6-1. APPLICATION
Vector aircraft:
a. In controlled airspace for separation, safety, noise abatement, operational advantage, or when a pilot requests. Allow aircraft operating on an RNAV route to remain on their own navigation to the extent possible.
b. In Class G airspace only upon pilot request and as an additional service.
c. At or above the MVA or the minimum IFR altitude except as authorized for radar approaches, special VFR, VFR operations, or by para 5-6-3, Vectors Below Minimum Altitude.
NOTE-
VFR aircraft not at an altitude assigned by ATC may be vectored at any altitude. It is the responsibility of the pilot to comply with the applicable parts of CFR Title 14.
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Center controllers (area) will never assign a VFR aircraft an altitude, they will simply tell the a/c to maintain VFR, therefore, they can not vector VFR aircraft for fear of turning the a/c into a cloud or some other factor that would make the pilot not comply with the FARs. Traffic advisories will be provided based on workload.