I've got a question on the vehicle damage by Incendiary Weapons. Do I still roll dice for the other weapons, and then get two effects?

Especially if I would put them with a unit with the tank killer ability. This would just be a waste if i could not roll another vehicle damage effect.

PeterClark said:

If memory serves, Incendiary automatically grants vehicles damaged by it the External Fire condition. This condition is applied automatically and is seperate from the roll on the Vehicle Damage Chart from the attack. If External Fire is rolled a second time, there is no change in the condition. You are likely to inflict two conditions to a vehicle when you attack with an Incendiary weapon.

Tank Killer is one of the effects that changes how all the dice in the whole attack work. Like Spray, Burst, Phaser, etc.,these modifiers affect the dice pool, all dice are rolled using the same modifiers. An attack that includes a Tank Killer weapon, all damage done would hit on blanks on the Vehicle Damage Chart.

Example:

A heavy walker is hit for 8 damage by an attack that includes a Tank Killer weapon. The armor save is made and reduces the attack by 3. The Walker takes 5 wounds and you roll 8 dice that hit on blanks on the Vehicle Damage Chart.

A heavy walker is hit for 8 damage by an attack that includes an Incendiary and Tank Killer weapon. The vehicle is set on fire from the successful attack. Tthe armor save is made and reduces the attack by 3. The Walker takes 5 wounds and you roll 8 dice that hit on blanks on the Vehicle Damage Chart. The vehicle is already on fire, so the External Fire effect does nothing, all the other effects work as normal. A vehicle may have every effect on the chart, as long as it is not destroyed before then.