P28 ECUs can be chipped to run a DOHC VTEC motor. The problem is that Honda changed the style of their ECU plugs starting in '92 and that's when the P28 ECU was introduced. You would have to hack up your stock '91 harness to run a P28.

The PR3 is a better option. It came in Japanese-market Integra XSi and RSi models, as well as the Civic and CRX SiR. These cars were all equipped with the B16. When using the PR3 there are just a couple of wires you need to add and it plugs right in.