Since when? She's a friend of the first 6 crewmates, but that doesn't make her a crewmate
Is she on the ship? No.
Does she travel around with them? No.
Was she formally invited onto the crew? No.
Did she decide to join them? No.
Is she loyal to Luffy as her captain? No.
Is she a pirate? No.
Vivi's no more a crewmate than Kin'emon and Momonosuke - they're allies and friends. Of course there's a chance this changes later, but she isn't a crewmate and will never be one unless she is invited to join and accepts, or asks to join and is accepted.
Anyone who considers her a crewmate must also consider a lot of other characters as crewmates. If being called a friend is the only criteria (which is the only way Vivi can be considered one) then you have to consider: The Akazaya nine, The Grand Fleet, Shinobu, Law and his crew, Carrot, Wanda, the other Minks, Pedro (rip), the Dold family, Koby and Helmeppo, Iva, Inazuma, Mihawk, Perona and all the other people who spent the timeskip with the crew, Franky Family, Galley-La, Crocodile, Daz Bones, Bon Clay, Shandian warriors, Aisa, Raki, Conis and Pagaya, Gan Fall, Pierre, Merry, Kaya, Usopp Piraes, Genzo, Nojiko, Cocoyashi Village, Jonny, Yosaku, Baratie crew etc. etc. to all be crewmates.
Even if we stretch the criteria as far as possible, i.e. good friend *and* having also travelled with them, then you still have to consider Law, Jambarl, Bepo, Penguin, Shachi, Wanda, Carrot, Minks, Nekomamushi, Inuarashi, Raizo, Kin'emon, Momonosuke, Johnny and Yosaku to be crewmates. Heck it's possible to even twist it enough to say Caesar too.