[Benjo]

GetVehicleVelocity is indeed a samp function. It will give you the velocity that the vehicle is moving on all the axis - x, y and z. You can read about it on the wiki here: https://sampwiki.blast.hk/wiki/GetVehicleVelocity

If you still have some problems, post the line/code in question and perhaps somebody will be able to help you out.