Coochy - hitting the airfield wasn't seen as a necessity.
The Chief of the Air Staff, Sir Michael Beetham, told the war cabinet that the RAF could mount a Vulcan raid on Stanley, but, that all things considered, he would only guarantee shutting the airfield with a minumum of 25 sorties, and preferably 50. He didn't think this necessarily the most practical approach, but if it was thought to be of help, then the RAF would crack on and do this.
The RN leadership, however, thought that a raid might very well persuade the Argentines to divert some of their Mirage IIIs to a defence of the homeland role, further weakening the ability of 8 Grupo to contest control of the air against the Sea Harriers. They therefore told Sir Michael that the raid could be of help, and would he mind awfully....?
There is, of course, a certain irony in this - for 30 years, we've had the joys of reading an array of comment - not all of it by Sharkey Ward - explaining that the RAF feared that without the Vulcan raid, they'd not play a part in the war (utter nonsense if you look at what the AT, AAR, MPA fleets got up to, and there were others, of course), and this would represent an existential threat to the service, etc, etc. Yet the most enthusaistic proponents of the idea of launching a series of Vulcan raids in a bid to achieve some sort of strategic/operational level effect wore dark, not light blue...
The net effect of this is that had the Vulcan not been available, there wouldn't have been an Op Blackbuck. The Argentines wouldn't have increased 8 Grupo's AD of Argentina tasking and more fighter sweeps - albeit limited in duration for want of AAR - would've occurred. Whether this would've simply offered the SHARs more targets, or whether it'd have made Corporate more difficult either through more air-air combat, or simply the presence of Argentine fighters more often prior to about 18 May (when the Mirage IIIs began to increase the number of fighter sweeps over the islands, albeit never managing to get their limited duration sorties to coincide with the SHARs' slightly less-limited duration sorties) is open to bar room speculation and nothing more.
Despite the Nimrod being fitted for 1,000lb bombs and CBU, I can't help thinking that the apparent lack of a strategic imperative to bomb Stanley at the time the decision to launch Blackbuck was taken would probably have seen any bombing of the airfield conducted by SHARs and the GR3s (as actually happened) with perhaps a greater weight of attack there had it become clear that the Argentines had taken steps to forward based A-4s/Daggers/SuE there.