I recently played a game against Plague Rat in Pike Complex.
The Vat that made all damage toxic pop up, then I was asked by the Expat. player if they equip incendiary ammo is that still fire type or toxic type? I went with the fact since the vat was out first, the ammo should override the damage since it would change the toxic damage back to fire. Is that correct? Not that it mattered since it was only 5 additional damage by Expat before the vats exploded.
This raises a question if say abilities like Ra's one that changes all to fire, or Twist the Either how are those ordered the same way. So you only take the last damage type modified by the newest card? Since they are applied from oldest to newest cards.
I don't think Incindiary rounds change damage type, and so it would still be converted to toxic. If a card like Ra's Imbued Fire was played after a vat then its damage type conversion would happen after the vat.
So now this begs the answer, so if say Shock Rounds was played. After hitting the initial target toxic damage from the gun due to vat will it do 1 lightning damage or would it be toxic instead since nothing else is changing that lightning damage?
I would say the lightning damage is changed to toxic damage - it doesn't say that any damage is changed to lightning damage, but that the Shock Rounds just deal lightning damage as-is. Therefore they'd be affected by the vat. If you had, say, Twist the Ether on you (from the Visionary), then you could change the damage type back to lightning (or to anything else).
It would be 2 Toxic damage to all non-Hero targets, since the Cyclohexane Vat provides a damage boost. And assuming no other modifiers, a Tactical Shotgun with Incendiary Rounds would do 6 Fire damage.