The burden of proof, in this case, should be on the e-cigarette industry. They are selling an addictive recreational product, consisting of a new combination of substances, with a novel delivery method (that involves deep inhalation). It would be naive to let them play the game the cigarette industry has played for decades, that is trying to dodge the burden of proof for safety and spreading uncertainty.
And let's not forget that, for a novel product, the baseline for safety is not using the product, not using something that is known to be extremely toxic.
All substances in e-cigaretes are harmless in dozes they are normally taken+. IMHO this is sufficient to claim that a mix of harmless substances is also harmless so the burden of proof is on somebody saying that such a mix suddenly became harmful.
+ I know that nicotine has LD50, so does water and even oxygen is dangerous at high pressure.
Conversely, caffeine is addictive, but is otherwise harmless (except in insane doses.) So long as you never try to quit, you'll never experience any negative side effects of caffeine addiction.
And let's not forget that, for a novel product, the baseline for safety is not using the product, not using something that is known to be extremely toxic.