I assume that your issue is trying to get the delta movement of the server player and the return result is (0.0, 0.0, 0.0) even though that player is walking around, however, the true delta movement is handled by the client player (or local player) at Minecraft.getInstance().player and you can get the delta movement from this. To get that from another player on the server, you can try using SimpleChannel (read this at Forge Documentation: SimpleImpl). By making the clients with your mod installed send to the server the local player's movement data and making the server send this to other clients, you can get the true delta movement of the players.
