Is there any burning or acrid taste or smell? You could be overheating the juice. Also, old style eGo vape pens, especially the top coil type, wick poorly with VG.
Previous heavy smokers sometimes complain of a "heavy lung" feeling from high VG, myself included. I don't use past about 55% VG.
For some reason there seems to be almost always an active thread concerning someone having health issues with a high VG unflavored vape. Usually they have a higher output device but that's the only difference. I suspect that unflavored is so unsatisfying that it is making matters worse somehow, perhaps increasing the tendency to run the devices hard, or maybe flavors have a more distinct effect when burned?