A Velocity Vector (VV for short) is a little symbol in the HUD (heads up display, although I am sure that is already known) that shows your projected path through the sky.
In other words, it everything were perfect and you planted the VV on a particular bush on the ground and kept it there, you would fly into that bush. It is a really useful toll for a lot of stuff, although there always seems to be an A (only the C models have it) guy who says it is stupid.
It takes into account wind where you are currently, coordination (like centered ball), velocity, vertical velocity, etc. It doesn’t react instantly, but gives a really trend indication. And if you line it up with the 0 degree pitch ladder on the HUD, you are level. Good piece of gear.