I only have one door on my car with this ventilation.
So to make the doors look the same im thinking of filling the holes for the vent in that door.
Is this a bad idea?
I think Plymouth quit useing it for -69 modells so i guess its not very important.
What do you guys think?

Sure, fill the hole, or cut a hole, which ever works if it is not a points show car. The vents were another case of Mopar over-engineering. I think the 67-68 barracuda's were the only A's that ever used them. May seem like a good idea on paper, but in reality, the cars were never air sealed tight enough for there ever to be an issue.