Agent, listen carefully. The vault's alarm will sound unless you enter the correct passcode within ten seconds of opening the door. You may use the eastern ventilation shaft to escape, provided that you don't mind dodging the laser grid inside it. Finally, If you encounter the villain's guard dog, simply offer it a dog treat and walk away slowly.

Unless means "if you don't." The alarm will sound if you don't enter the passcode.

Provided that is used to say that one thing must happen or be true for another thing to happen (you can use the exit only if you are okay with the lasers).

If introduces a general possibility or condition. We use it here to tell the agent what to do in the specific event that she sees the dog.