Just my view on a couple of things I read in this thread:
Vaping is, in my opinion, not very likely to cause cancer. As another poster said above, nicotine can interfere with apoptosis so it is possible that it may increase cancer risk, but is not carcinogenic by itself.
While vaping can cause mouth sores(as can nicotine gum, acidic or spicy foods, biting your cheek, etc), this is also not likely going to increase cancer risk. Nicotine along with many other substances can irritate or damage the mucous membranes in the mouth, causing sores. This cell damage is usually quickly repaired by the body's natural healing processes.
Long term safety is unknown and has not been tested, but by looking at the contents of e-liquid vapor is not likely to cause major problems for most users. For users who switched from normal cigarettes or cigars, an e-cigarette is a much healthier alternative, however not using the e-cigarette is likely to be the healthiest option.
For those who worry, or wonder, or might be looking for ammunition to deflect future arguments, consider the food we eat. Your average person consumes food that is packed full with artificial colors and flavors, and even fruits and vegetables are awash with fertilizer and pesticide residue. Someone wanting to make a healthier choice may start cooking their own food, or buying organic produce, but even those choices aren't 100% safe - but they just may be better for you than what you were originally doing.