By "NIPS", are you referring to a network-based intrusion prevention system? Depending on the setup, integrating an IPS into the network is one rare time when you may choose to break the cardinal rule of one-to-one vlan to subnet mappings.
To answer this, can you be more specific with your question? What does your network look like now, and what are you trying to do? There's more than one way to set up an IPS in a network, so I'd need more info on what you're trying to do.
CCNP, CCDP, CCIP
Core Network Planner, ISP