What do you do when the thief is in the vents on night watch island?

Follow him and use the blowtorch.

When you are in the vents there will be red bars between you and the thief. Use the blowtorch to cut through them and climb up the ladder. Then go to your right, and do the same thing. Once you end up near the machine where the elevator and escalators work, you will see him again. Once again you have to use the blowtorch on the bars. When you catch him in the air vent chamber, he knocks you away.

Go back right to the vertical shaft, burn through the brown bars, and destroy the A/C control so that he has to move. Go back into the air chamber, across to the spinning fan on the left side, and burn through to Vent B. When he blocks your path, go left onto the dark maintenance ladder. When you catch him, you both fall out into the atrium fountain.

Eddie the snake saves the day, and the cops are waiting for the thief.