When I bought my VFFs, I tried them on without socks. I recommend you do the same.
I prefer to wear the Injinji toe socks with my VFFs. For the first few weeks after purchase, I wore them without socks, but eventually found that my feet would sweat (it’s hot here) and my VFFs would stink. They’re machine washable, of course, but that’s a pain. So I got the socks and that has helped with the sweat/smell problem quite a bit.
If this will be your first pair, I strongly recommend that you try on VFFs at a store to ensure you can fine tune the fit.
Note: if your second toe is longer than your big toe, then you can’t wear VFFs.