Vehicles don't fight in close combat as such although they can be attacked by enemy troops. However, heavy vehicles like tanks can break through enemy positions, scattering units and possibly forcing them to fall back – this kind of attack is called Tank Shock and is described below. Dreadnoughts, giant humanoid walking machines designed to spearhead assaults, and other walkers fight in close combat the same way as troops do.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): If a squad makes a sweeping advance into a vehicle on its turn, the vehicle may fire at the troops as they assault as per the normal sweeping advance rules, or it may move away (in its normal Movement phase) before it is attacked.