Does a Peak or Average Power Meter measure All Harmonics? or limited by VBW?

When you have a 2-tone input in a PA for IMD testing in MWO, the output power can be either Total Power (PT), Fundamental/Harmonic Power (Pcomp) or the total power in a certain bandwith PTB.

For my optimum load impedance design for maximum output power, i use the Total Power (PT) which includes all harmonics + DC.

My question is when using a Peak Power Meter or Average Power Meter instrument, does it actually compute the Total Power (PT) ? Assuming the input is a digitally modulated 5MHz OFDM signal.

You should know your power meter. Read its specifications and find application advice by the manufacturer.

Power meters come in specific versions. Those with a coaxial input measure the power by heating sensor that is "linear" from e.g. 10 MHz to 18 GHz, so any harmonics falling within will contribute to power reading.
Microwave waveguide sensors are calibrated over the waveguide band, and harmonics falling outside may or may not contribute to reading. Low-pass filters are recommended to be sure.

Peak power meters typically use calibrated detectors for a fast response. Again, harmonic response should be included in calibration.