I'm getting away from the original question but eeper mentions the legality of a clearance that puts a VFR aircraft 500ft vertically away form an IFR flight.
This all depends on the class of airspace. If I recall correctly, in ICAO terms in Class D or below, there is no requirement to separate VFR from IFR flights. The only obligation on the controller is to pass traffic information although here in the UK it is quite common for controllers to restrict VFR traffic so as to ensure that there's 500ft vertical between them and an IFR flight.
Specifically on the TCAS question, is it worth mentioning that you'll only get a RA if the other aircraft is squawking mode C - if it's mode A only, all you'll get is a TA.
There endeth my knowledge of TCAS!