Succatash, the operation of TCAS (both I AND II) is dependent on the "target" traffic having a mode C transponder operating. Earlier in the thread it was indicated that the 421's wasn't and it was only when he asked for an IFR upgrade that instructions to "ident" then select mode C were issued from ATC. It would have been then and only then that the
VB aircraft would have gained SA on this guy
Whilst he was legal to be there, good airmanship would dictate that it wasn't prudent, given the proximity to a busy terminal area and IFR tracks to said terminal.
Similarly, the dependence on the "see and avoid principal", which I will agree is completely flawed, even for individuals with Yeager-esque eyesight, was made a mockery of when the 421 pilot indicated that he had dropped that scan to follow the instructions issued by air traffic. ASA take note!