Actually a P28 only has one VTEC output, because VTEC works the same on DOHC or SOHC engines, there is just an extra cam for the DOHC, etc.
The P72 has the extra output for the IAB (intake air butterfly/secondary runner control, whatever you want to call it) that the P28 doesn't have.

Put it this way. A chipped P28 will run a DOHC engine great, but a P28 doesn't have a knock sensor. A lot of people don't care about that.
Actually the only difference between the P28 (SOHC) and P30 (DOHC) ECUs is that the P30 has a small extra circuit board in it for the knock sensor. They're the exact same ECU otherwise, except for the program.
P28s have a SOHC program in them, so get a P28 chipped with a DOHC program and you're set. Easy.