Friendly vehicles never block LOS/LOF for other friendly vehicles.
Operable enemy vehicles block LOF, but not LOS, from friendly vehicles.
Non-smoking KO'd vehicles do not block LOS/LOF for friendly or enemy vehicles.
Non-smoking KO'd vehicles block LOF, but not LOS, from any unit as long as the targeted unit is not a vehicle (ie: tank shooting at infantry or infantry shooting at infantry)
Smoking vehicles block LOS and LOF.
"Vehicles" means tanks, SP guns, and AT/Anti-personnel guns.