Generally, when a tube red-plates, it's due to the signal, bias, or plate voltage becoming too high. This causes the tube to dissipate more power than it can handle causing the tempurature increase, hence the red plate. This can be caused by a bad tube, bad transformer, faulty solder joints, faulty capacitor(s) and/or resistor(s), or higher signal levels than for which the circuit was designed.
Which tube is red-plating? Is it a 12AX7 or is it one of the power tubes (EL34 or 6L6)? While two amps experiencing the same failure at the same time is unusual, it's not impossible.
Since you've said that the amps doesn't do this with any other pedal, or other effects with the PF (just the octaver), I'm wondering if the octaver is producing a whole bunch of frequencies that the amp can't reproduce, which is causing the over-dissipation.
If you have a scope, you could check this directly. If you don't, some DAW software that contains a frequency analyzer could give you the same answer (you'd have to plug the PF into your computer). This could help determine whether the problem is in the pedal or in the amp.
This could also be caused by a physical defect in the effects loop or bias control section (your amp has selectable EL34/6L6 bias and a half-power switch, doesn't it?). It could be that there's a physical defect in the pedal, too, or maybe the interconnection cables. My first guess would be one of the PCBs in the amp, though. My first instinct is that the problem's always there, but doesn't cause the red-plating issue until it's hit with the lower frequencies coming from the octaver.
Have you ever plugged a bass or other low frequency source into your amps' effects return? That might be a way to test the above theory. If this is the case, it could be improved with the addition of a high-pass filter to the effects loop (the schematics I've seen for your Carvin Vai Legacy don't contain one, that's not too unusual though).
In any case, what you're describing is not normal behavior (as if I have to tell you that) and it sounds like, at least to me, that there's a physical defect in your specific amp or pedal which might be better diagnosed by a qualified tech.
I'm sure you know this, but I'd be remiss if I didn't include a disclaimer that poking around inside a tube amp can KILL you, no joke, or seriously injure you. So, unless you're qualified or have a death wish, it's definitely a job for a professional.