I've to say that I'm Argentinian and if you dont mind I'd like to offer some input on the subject.
I had a meeting with engineers recently who were based at FAA Rio Grande Base during the war, supporting the Daggers, and they told me that they performed some touch and gos with the Mirages to seize the possibility of using Stanley as an emergency runway for them. They never planned to operate from Stanley for the resons already posted.
As for the Vulcan mission, yes, it was there, did that but every argentinian book about the war emphatizes more in the labor of the Harriers in their bombing duties and the job of the battleships constantly aiming the runway. I honestly don't know what the junta was thinking on the matter, but to my knowledge there were 3 MIII in BA to protect the city even before the war started per se.
We're actually more intrigued by the SAS chopper mission to Rio Grande which ended in Chile and written off. There're lots of stories about that night but nothing concrete. Better not talk about it I guess.
Of course nothing can be 100% spot on, but I wanted to give my view on the matter.