Pre 92 hondas are obd-0, including the pre 92 VTEC engines. OBD has nothing to do with vtec.
From what Ive read on the codes, the OBD-0 systems are very basic. For example our crx's codes are pretty simple, code 1, 2, 3 and so on representing where the problem is. Here is a LINK to codes for our obd-0 cars. Pretty basic info.
OBD-1 and 2 are far more advanced. Instead of a basic idea of the problem you can get codes for a misfire in a particular cylinder, or voltage in a circuit being to high or low, so its a little more detailed.
Heres a LINK scroll down a little and you'll see a list of obd-1 and 2 codes