Its a drian rather than a vent, if you have a sunroof you should see two holes at the front either side of the roof channel (with the window fully back)
Sometimes these get blocked, so worth checking visually for any blockages, but IIRC to access the pipe in the A pillar you need to removed a few bits of plastic trim... I'm sure someone will explain if I am wrong though
A pillars are really easy fingers in and pull off.
Also once trim off, a good chance to clean out any foreign objects....I had a load of rotten leaves in my scuttle trim...They had gone very mouldy!!!!!!!!!!!!
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