Really does not work that way as the numbers need to be accurate on all the instruments to give you a VMG that is worth sailing . if the speeds are off on one instrument and not on the others then you will think you are sailing to best VMG. In reality you are sailing to what the instruments say is a best VMG but could be way off the mark. Garbage in Garbage out
I don't think it works that way. Even with mis-calibrated instruments, or one mis-calculated instrument and one good calibration one, the relative
VMG will still be correctly optimized - just not accurate. In other words, your boat will still be sailing its best VMG, but the actual VMG value may not be correct.
For example, say the wind instrument is off by 5*. When you are sailing at optimum relative VMG, you might be making 5kts at 50* TWA by the instruments - where in reality you are making 5.5kts at 45* TWA.
In either instance (poorly calibrated, or precisely calibrated), if you fall off or head up from the optimum VMG, you will no longer be at optimum VMG. Whether you are actually making 5kt or 5.5kt VMG is inconsequential - it is still the optimum point of sail in those conditions and trim.
This becomes particularly true when using SOG, which is always calibrated.
Edit: I see you mentioned the need for accurate speed values for all
instruments. I'm not sure why one would use multiple boat speed instrumentation to calculate VMG, but even if one did, the relative
optimum VMG should still be the same.