Vierge means "virgin" which I suspect refers to the olive oil used in the sauce. How this translates to a butter based sauce is anyone's guess.
My 1977 version of Larousse doesn't have it at all. Michel Guerard has a recipe for Sauce Vierge in his 1976 'Cuisine Minceur' book that he also put in his book 'Cuisine Gourmande.'
His recipe is a bit more elaborate than that of more recent times. It also doesn't include either basil or lemon juice. His recipe has: tomatoes, garlic, chervil, parsley, tarragon, coriander seeds, fromage blanc, mustard, worcestershire sauce, and olive oil.
The use of olive oil rather than butter in a French sauce suggests to me that the concept stems from Guerard's Cuisine Minceur.
Michel Roux's version is closer to yours and I suspect that this is the one that has been picked up by the UK chefs and wikipedia as being "genuine."
