So is Guerlain a niche or a designer fragrance? Guerlain doesn't make clothing and lots of other "designer" goods that most designer fragrance houses, do, although Guerlain does produce makeup, and some Guerlain frags, including Vetiver, are widely available, including at most department stores.
I don't think of Guerlain as niche. Like you said, some of their fragrances are available in department stores which is not the case with niche houses. Whether that places them in the mainstream/designer category, I'm not sure.