Vash,
That is a great question and one many people are trying to answer; at the end of my post is a link from The Guardian newspaper in the UK.
Vaping is a little like eating, you can buy oranges from a guy at a freeway exit or you can get the organic ones from Whole Foods; you pays your money, you makes your choice.
Until quite recently (last 18 months) most over the counter E liquid or juice came from China. Not to impugn anyone there but the Chinese track record on baby formula, drywall, pet food and probably e juice is frankly, terrible.
If you are smart and care about what you put in your body you can buy all natural or organic e juice made in a lab by a Stamford grad in a white coat. Or you can go online and buy a plastic bottle full of something made in that morning in a bucket.
As with anything, there are extremes and one would be wise to educate oneself before ingesting anything that may do more harm than good. The following article comes from The United Kingdom, before we turn this into a debate as to relative merits of The National Health system in The UK, please bear in mind I grew up in The UK, both kids were born there and I find my medical experiences there as good, or better, than those I have had here in US.
Vaping: e-cigarettes safer than smoking, says Public Health England | Society | The Guardian