RPE is a good tool for judging your effort while exercising. For the VO2 test, you really just need to look at HR. The RPE should correlate, but a lot of people need to "calibrate" their RPE scale to be able to use it well.

As for low VO2: it's mL/kg/min, which means that since your weight is high, your VO2 max will appear low. It is quite skewed by weight.