Use the money spent on the first 34 Sea Harriers of OTL on an upgrade to F Mk 3 standard which enables them to fire Sparrow or Sky Flash instead of Red Tops. Although they don't have a gun they do have a pair of retractable pods containing fourteen 2" rockets in the front-fuselage.
Sounds like a good plan.
With Gannet AEW in play, the Sea Vixens should be able to be at the right altitude and place to counter their power/weight deficits and thus intercept Argentine fighters, Skyhawks and Super Etendards.
My thinking is with only two old and small CATOBAR carriers focused on ASW, plus one LHA (Albion), the same impression that Britain doesn't care about, nor can protect the Falklands should still be in place in Buenos Aires. By the time that impression is shattered, Hermes and Bulwark are enroute with Sea Vixen, Buccaneers, plus Gannet AEW, while Albion is packed with RAF Harriers, plus Chinooks, etc, plus many RMs.
Sea Vixens can escort the Black Buck missions, assuming with the Buccaneers in play they're still necessary.