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I'm just starting to paint up a crimson fists army. I know that the infantry do not display company markings but I was curious about the vehicles and wasn't able to find information on them.

If infantry don't, then why would they bother with tanks?
If you want to do it, then it's fine.
If you don't want it's justified.

If you look at the line art of the Crimson Fists in the Space Marine Codex (p.92) you'll see that the dreadnought pictured has a Roman numeral denoting its company (the Fourth) and its place in the company (it is the seventh vehicle). I don't think you can go wrong whatever you do as there is sufficient flexibility in the game lore to justify any combination of heraldry and makings, but the 'default' lore seems to be to use company and vehicle numbers.

My Crimson Fists represent the Fifth Battle Company so what I do is put one Roman numeral 'V' on every vehicle to denote that it is attached to, or organically part of, the Fifth company. For vehicles attached from the armory, like Predators, I also put on numbers designating its place in the squadron('I' through 'III'), while transport vehicles organic to the company get Arabic numerals to indicate what squad they are assigned to, as my infantry all have Arabic numerals on their shoulder pads and the consistency pleases me.

For what it's worth, I also give my sergeants red helmets as nod to the Codex markings, even though I can't find any information on whether Crimson Fists give any sort of special markings to their sergeants or not.