The Legend, as I heard it, is that The Perfumer's Apprentice was a distributor of liquid concentrates designed for the Foodservice Industry. The earliest DIY Vapers went here for the flavors they were trying to mix, but those products were made for digestion. The manufacturer quickly began reformulating for inhalation designed products, and called it The Flavor Apprentice.
The DX line of TFA is designed for Vapers who wish to avoid diketones all together. Some say they are safer, others say they don't taste the same. To Each their own.
I personally am satisfied with the DX Vanilla flavors (Bavarian Creme, French Vanilla) and use them in all of my liquids that call for it.