It also depends on what scale you want to play.
If you are playing 1:1, i.e. 1 stand = 1 truck etc, then it's up to you if you leave the destroyed truck on the board or not. At 1:5, (1 stand = 1 platoon of trucks), the it becomes a bit more abstract, as a knocked out unit won;t necessarily mean 4-5 burning trucks...
Personally I removed destroyed units (inf and vehicles) as it removes clutter from the table (I play at 1:5).
If you leave the destroyed vehicles on the board, then it will be up to you and your opponent if they offer cover or not.
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