[Calico]

As a combination of what others have said, it's part "this is her style" and "ayu is a fashion icon". Ayu is often used to push certain brands, and even though her music may not be as popular now, many young girls still see her as a huge fashion icon. There are some girls I've talked to who see Ayu as a model first with that whole singing thing on the side, lol.

ETA: OH, and the reason for Vivi specifically is two-fold. 1) As someone else said, there's the deji-deji thing. Plus Vivi often features the "queens" on their covers. Namie is another super popular one, and sometimes Kumi is on there too. Kana and Miliyah are rather popular and I've seen Anna on there at least once. and 2) vivi is one of, if not THE biggest Japanese fashion magazine. So it's actually a really good thing that she's featured a lot, because they pull a lot of weight. Vivi is always pushed to the front of magazine racks and is, imo, one of the more expensive ones lol (as I was reminded when I bought the most recent one tonight).