Cork to Vigo would be about 30 hours, but it's also going to have to stop in Plymouth so that's several extra hours sailing with a few more for unloading/loading. I can't see how a ferry going to Plymouth and then either Cork or Vigo can take the same time as a direct from Cork-Santander or Rosslare-Bilbao.
Perhaps it is intended to operate from Cork-Vigo-Plymouth whereby; the Cork to Vigo element is expected to take up to 28 hours?
Otherwise, if it is planned to go from Cork to Plymouth to Vigo, I've no idea why they would have referenced "up to 28 hours" in the article published in CorkBeo.ie recently.
I would imagine that the Port of Cork will not want to see a situation of yet another ferry operation being set up on a trial basis only to be axed within 2 years coming so soon after the Cork Santander service was cancelled and switched to Rosslare Bilbao instead. The additional loss of the mid-week Cork Roscoff sailings which will depart ex-Rosslare from March has also been an additional blow for the Port of Cork and Ringaskiddy Ferryport not to mention the effect on Cork/Kerry tourism.
Until the proposed Cork Vigo route is officially confirmed along with the ferry operator, this is all just speculation.