These acidless oranges are apparently greatly appreciated in several North African countries, particularly by children.
I bought one from Italian grower Tintori a few years ago, specifically with the objective of trying to cross it with some of the hardy trifoliate hybrids. The idea was to eliminate some of the bitterness and acidity of the hybrids. However, when I tasted the fruits of the Vaniglia orange I had bought, I found they were no different from other oranges. I still don't really know whether this is because the variety was incorrectly supplied or that my cool UK growing conditions do not favour the development of acidless fruit. So the hybridisation experiments never took place.