Logically, vatted malts have the opportunity to surpass singles in quality.
I understand the "logical" concept - afterall, you should be able to combine all the good bits of various malts and make a "super malt"!
But it just doesn't happen, does it?
When people are asked to reel off the best tasting whiskies, they immediately identify the famous single malts - a particular expression of Macallan, a particular vintage Ardbeg, a certain Springbank, a well-aged Glenfiddich, etc, etc.
If vatting truly did produce superior tasting whiskies, then why aren't they being made and sold in abundance?
If you start with a mediocre malt, and vat it with another mediocre malt, it does not logically follow that the combination will produce anything significantly better than mediocre.
And if you started with an absolutely fantastic malt, and vatted it with another spectacular malt, it does not necessarily follow that the two (or more) will marry together and produce the greatest whisky ever made. In fact, it is more likely that the complexities and subtleties that made the component single malts great in the first place will actually be lost.
At the end of the day, it comes down to the skill of the blender (the vatter?) and the component whiskies he has at his disposal.
But do you seriously believe Macallan (and any other "premium" distillery) would sell one of its finest casks to go into a vatting where its identity & provenance will be lost? If the cask really is that good, they will sell it as a distillery bottling under its own name so that its glory will reflect on them.