As far as I know, we don't have any real evidence that nicotine in and of itself causes cancer.

Nicotine in ejuice is largely extracted from tobacco, and depending on its source could easily still contain nitrosamines, which are pretty potent carcinogens even in trace concentrations and you still get some small amount of combustion with ecigs around the wick, which would be carcinogenic, too. Of course, that's likely to be a relatively small component of what we're exposed to in an industrialized world anyway.

That said, stimulants of all stripes do encourage the growth of existing cancers, though, even if they don't initially cause them; definitely something to think about for former smokers, especially.

I was thinking of cardiovascular health, and particularly atherosclerosis, which has a definite link with nicotine specifically even in the absence of everything else in smoke; although a quick pubmed search seems to suggest that the risks are apparently less than I remembered.

It also seems there's some moderate evidence of adverse effects on fetal development from nicotine alone.

Personally, I suspect the effects of just nicotine in propylene glycol/glycerin on your health are relatively small. If it winds up being the case that ecigs have major health effects I'd put a large wager on it being the flavourings that are at fault (which is why I personally go with unflavoured ejuice).

Even so I'd recommend a non-smoker who wants to take a mild stimulant just start drinking espresso instead. Or, if you're dead set on nicotine for some reason, it'd probably be wiser to like, add a few drops of ejuice to your coffee instead.