But to answer your question the Infantry/Vehicle Command skills will decide how many squads of NPCs/vehicles you can control at once, and the combat skills will determine how well you can use the onboard weapons of the vehicle you are driving. Presumably once NPC pilots are implemented their skills will determine how well the vehicles they are in will fire.
NPC means Non-Player Character. Basically that they exist and can perform functions based on how you employ and interact with them as a PC (player character, you). In simple terms, the game governs them while in your case you govern your player.
The portion of the rules dealing with NPCs can be found here.