If you're concerned about safety, then you're probably best off sticking with unflavored juice. The food-grade additives used to flavor e-liquid are meant only for ingestion and are not rated for inhalation*. There's a whole group of them (called diketones) that are of questionable safety and at least one in particular (diacetyl, used to create a "buttery" or "creamy" flavor but also naturally occurring in many other common foods and flavorings -- even some of those labeled "no diacetyl" only means that none was added to the juice but it may still be present in the "natural and artificial flavors" from which it's made) that is known to have caused irreversible lung damage (look up "popcorn lung") when inhaled in large quantities by workers in the factories where microwave popcorn is made.
Even discounting those, however, the fact is that we just don't know what is safe to vape and what isn't. vaping is still in its infancy, and we simply don't have the long-term data like we do from centuries of tobacco use. In ten or twenty or a hundred years it may turn out that diacetyl is the least of our worries and it's actually the fruit flavors, for example, which are the real killers after all.
*Menthol is considered safe for inhalation, and propylene glycol has been used in medical inhalers for years. You could probably vape menthol-flavored PG-based nic juice (or non-nic juice, if that's what floats your boat) and still be generally assured of relative safety. Anything beyond that, however, is a total crapshoot. If you're already hooked on the nic, or you just crave that hand-to-mouth action, then it's (probably) a safer alternative to combustible cigarettes, but nothing that you put into your lungs is going to be as safe as pure, fresh air.
