The phrase 'designer' is an adjective but also a job description. As someone else pointed out if it had the head/chief designer/CD in the title of the brand it's considered designer, even if it's a diffusion/younger/sports line. That's the business definition and that's why you'll find most of these names in the designer/gold rails in T K Maxx etc.
Vivienne Westwood is a designer that no longer designs a great deal, her name, silhouette, and brand's ethos are interpreted through the seasonal collections by the current designer Andreas Kronthaler although she oversees the creative vision of the overall brand.
In the wardrobe world, Alberta Ferretti is most defiantly premier designer. She has had diffusion lines in the past (Ferretti) which was also fairly expensive. A full-length gown at Ferretti can cost around $3-8K.
The reason Ferretti is not on 'Vlad's list' is that AF is a gown specialist that does other RTW, not a leather goods brand. All RTW is together on 'Wardrobe & Accessories' whether that means Zara or Loro Piana.
There are some brands that started off as couture fashion/RTW and diversified into accessories (where the real money is) e.g. YSL, Valentino and Alexander Mcqueen and then there are leather goods/saddlers/trunk makers that incorporated RTW (for the fashion show publicity and seasonal direction) e.g.
Gucci or Hermes. These are both very different beasts. Most of YSL's and Mcqueen's bags and leatherwoods are made in Gucci's factories, whereas the silk scarves that Gucci, Valentino, Mcqueen and many others sell are designed in-house and then made in the same factories in the Italian lake district. Hermes makes its own scarves, many of them in Lyon but some in other parts of the world too.