Only on IPS 4200 series appliance you can configure either interface mode or inline vlan pair. On AIP module, there is no concept of interface pair as the sensing interface is through the backplane of the ASA firewall. On IDSM module, there is no interface as well, so only vlan-pair can be configured.

If you have multiple vlans that you would like the IPS to inspect the traffic, then you would use vlan-pair deployment. However, if you are routing everything through the IPS then interface pair deployment should be used.