Open the air shutter on a gas burner so the mixture of gas and air has more air you'll get more soot; less air you get more fumes. The balance gives us a clean burn. If by vent pipe you mean termination cap the pull of the chimney is usually enough to keep fumes and soot moving out but every chimney cap is black with soot. If this vent runs horizontal then it s a direct vent fireplace and these burn dirtier than other vented fireplaces. Propane burns clean but does still produce soot and poisonous fumes. Vent free gas burners are the cleanest because the BTU is lower and flames do not touch ceramc logs but even ventless fireplaces need their firebox hood or soot will stain the mantle.
Yes, LP burns clean but this still means there will be soot and fumes. Soot is normal. If you're concerned test the gas pressure, have a tech test the pilot and air flow to ensure maximum efficiency... But most vented fireplaces will not burn maximum efficiency like unvented burners because their is a trade-off in heat and beauty.
Soot being emitted from the outside vent pipe in a magnitude that it covers the roof drain indicates a problem with the fireplace. Furthermore, the excess soot is a key indicator that the propane is not being efficiently burned. You should get the fireplace checked out by a professional before the problem worsens.