There's a small chance for them to drop when opening Commendation Chests, but relying solely on this approach won't get you too far. The most reliable source of Red Items in Vermintide 2 are Vaults earned by completing missions on the Legend and Cataclysm difficulties.

Can you get veteran gear from Commendation chests?

There is also a small chance that a Commendation Chest will drop a Veteran item.”

When should you open Commendation chests?

You should always open chests on your highest level character. Your “max power seen” across all of your characters factors into the loot you receive as well. Your loot will be an average of your “max power seen” for each slot, meaning the max power of the item will be -5 to +10 of your max power seen.