Are these socks thin enough to be worn with Vibram Fivefinger shoes?
It depends how snug your Vibrams are. I have some KSOs they won't work with and some KSO Trek's that work great with them. Definitely get a snug fit with the socks. Downsize if you are on the cusp.
Yes they are. II wear them in my treksports and they fit perfect. Just keep in mind that a new pair of vibrams will fill too tight with injinji socks. But once they have been "broken in" then you will be fine.