Hi, Vivi is a well sought after card, with many people being fixated with it. I sold one this afternoon because I have too many Magic units and I would much prefer a missile or melee unit to it.
I posted my MAXED Vivi for 300 elixer (Which is a fair price for I can actually sell it at 350) and 5 mins later, somebody approached me and bought it. If you can MAXED your Vivi, you can command a higher price but otherwise, I last saw a lvl 1 Vivi going for 240 elixer.
Don't ever trade it for just Fiona (Especially), Thrian, Sera or Gerhard alone. Good luck!