There is something I don't understand. I have a macbook pro 15' retina display.
The information says it has a
2880 x 1800 display but when I open a 1080p video it covers the whole screen. How come?
This was probably done so that videos, especially those that are lower resolution than 1080p, don't look absurdly small. If you want to play your video at the native resolution on your screen, (and you've selected "best for retina" as your display resolution), you could always tell VLC to play it back at half the size.
Make sure that you're using an up-to-date version of VLC. Retina support was only introduced in version 2.0.2. If you're running a prior version, it may end up pixel-doubled.
If you do have an up-to-date version, make sure that the video window hasn't just been scaled up. You can resize it to normal dimensions by clicking Normal Size in the Video menu, or pressing ⌘1.