Ecigs don't heat anything to the point of decomposition. They certainly can get very hot, if there is no liquid left to cool the heater coil; but if there is no liquid left, it can't decompose
And if you've ever tried going that far, you'll know it is impossible to inhale the smoke (and it is smoke).
On the subject of bulk VG or PG deals, please remember that what we are doing here is inhaling large quantities of materials over a timescale of decades. For that reason the question that I personally ask is, "Where can I buy a more expensive e-liquid, that has been tested better?".
Vaping is so incredibly cheap compared to smoking that saving the last cent on what you will be inhaling for decades doesn't make a lot of sense to me. Unless you are a collector/enthusiast it can't cost more than 25% of smoking. Shaving a few cents is not logical to me.
VG and PG can be produced in four (or more) grades:
Industrial Grade
Agricultural Grade
Food Grade
Pharmaceutical Grade
You shouldn't inhale Food Grade never mind any of the others. If you knew that Industrial Grade (for antifreeze etc) contained 5% of toxic contaminants, would you still inhale it? Do you really believe that someone can sell Pharma Grade (pure with no toxic contaminants) for $5 a gallon (or whatever it is)?
Please exercise due diligence here - you will be inhaling this material for years or even decades. You won't die early from cancer, lung disease, heart disease and so on from smoking. But someone, somewhere is going to die young from toxins in the refills - and I suggest you take steps to avoid it being you, if you can.
