I am replacing deck around a vent pipe. Can I just cut a hole in the roof deck or do I need to reinforce it somehow?

It depends on the size of the hole (or dent, if the deck was damaged).  The Steel Deck Institute provides the following guidelines for reinforcing 1.5” deep metal roof deck based on the size of the opening.
Reinforcing steel deck when there is a hole in it
  1.  Up to a 6” diameter and no more than two vertical webs are removed: no reinforcement is typically required.
  2.  6” to 8” diameter: cover with a 0.045” (18 gage) plate that extends to adjacent ribs.
  3.  8” to 13” diameter: cover with a 0.057” (16 gage) plate that extends to adjacent ribs.
  4.  Over 13” diameter: the opening should be framed and the project engineer is responsible for the design.
covering hole in steel deck with flat steel sheet
Exceptions

1. If the deck can safely be designed as a cantilever from each adjacent support, it may be possible to eliminate the need for reinforcing openings larger than 6 inches.

2. On the other hand, if a group of holes is close together, a structural frame may be required.

The project engineer should be consulted where these exceptions apply.

As always, please ASK D-MAC when you encounter field conditions that might impact the strength of your metal deck. We are ready to help!