minic6, you are a working fool.
I would be concerned that the air flow past your vent would be disrupted by the low pressure created in the wheel well by the spinning tire. The higher pressure may try to enter the vent from the back. That would not be good. But you will still have some high pressure air entering between your work and the tire, maybe enough to generate flow out the vent.
Leave it for now and, if you want, I will loan you one of my pressure/flow gauges to try and analyze what's happening. You can test it as it is then tape it over to test it closed. I may even have a suitable spare gauge.
When you get the one side finished enough to test, the gauge would be very useful in getting an idea of what you have down the whole side so far. If changes need to be made, you can figure out what they are before starting the other side.
I would hate to see you make a cutout in the rear skirt. Do you think it would stay if you blocked the fiberglass out an inch or so from the hubcap and applied more resin/cloth? After trying that, I would consider flattening the hubcap a little at the center. It will be hidden anyway. You have a pretty nice aero shape going and, to my eyes, a cutout would spoil it. You are extending the rear skirt down to the bottom of your flare, are you not? Plan for what might happen if you have a 100% flat tire. Better now than later.
Be careful breathing all that stuff. Pretty mean on the respiratory system.
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