There can be a lot of factors here. Your mouth is a complete microbial ecosystem, containing a wide array of microscopic flora and fauna. Propylene glycol and (to a lesser extent) glycerin -- which are the predominant carriers in electonic cigarettes -- both have germicidal qualities. Not all microbes are equally affected, so this can alter the relative populations of various microbes in your mouth.
It just so happens one of my sideline hobbies is studying the microbes (mostly bacteria because I'm best equipped to study them) throughout the digestive tract from entry to exit, and one thing I have discovered is that which microbes predominate in a region is affected by many factors. Interestingly, also, the sorts of compounds created by those microbes will also be affected both by their relative predominance and other factors in the environment such as changes in diet.
This is a long way of saying that the reason, I believe, why many people suffer from soreness in the mouth, tongue and throat when they first start vaping is because the PG and/or glycerin alters the balance of bacteria in that region and also alters the environment in which those bacteria live. This causes a temporary shift in which some of these usually benign bacteria take on near-pathogenic qualities until a new equilibrium is established.
This same occurs in the intestines when a person makes a radical dietary change such as eliminating all carbs. Bacteria that were previously thriving on carbs in the intestines experience an altered environment, and generate compounds for a short time, until a new equilibrium is established, that will even make the person who changed diet feel sick. There is a lot of research piling up indicating that intestinal bacteria exert indirect influences on behavior in order to pursue their own interests independent of the larger organism they inhabit.
Unless you are having an allergic reaction to the specific ingredients in your e-cigs -- which is rare but possible -- the cause is likely what I am describing.
Given time, it will pass.
One trick that naturopaths and some nutritionists use to reduce these problems when changing diets is to reduce overall intestinal bacteria populations. (Bacteria make up 40% of the dry weight of feces.) They do this by having their patient fast for a couple of days and take a laxative prior to undertaking the new diet; and then augmenting their first portions of the new diet for a couple of days with neutral bacteria that have not yet developed a specific food preference. This reduces the overall intestinal bacterial population and replaces some existing strains with bacteria that will be more friendly to the new diet, and thereby improves the odds of long-term compliance of the patient.
So this suggests a similar approach *may* be useful for vaping. Unfortunately, I have not studied the differential in effects of PG and glycerin on common mouth bacteria.
An analogous approach would be to use an antibacterial mouthwash (such as listerine) twice daily for one minute -- say, in the morning and before bed. This accomplishes the same thing as fasting and purging. Then, populate more neutral strains through a broad spectrum neutral digestive bacteria supplement except, instead of swallowing it, break open the capsule and add it to some water that you swish around in your mouth a couple of times daily. This would help establish a new environment with a bit more rapidity.
But, if the above sounds like a pain in the tush, I would say just ease into vaping by at first using it to replace every other cigarette and then increasing so your mouth environment can adjust gradually. Either way, eventually the problem goes away.
As for using the disposables, imho, they are mostly horrible. I won't name names, but many of them are bad. As madjack notes, maybe dormouse will have some specific helpful suggestions in terms of gear. I also think madjack's specific recommendation of some easy cartomizer-based e-cigs is a sound one.
