I've been vaping since Nov. 2013, and I've only had vapor's tongue 2 times...and that was in 2014, for about 3 days each time. For about the last year I've been vaping the same DIY eliquid all day, every day. Sometimes I'll vape about 1mL or less of a different one, maybe once a week at the most, but that's it. So for me anyway, and possibly others...vaping the same eliquid instead of a variety of different ones, isn't what causes vapor's tongue if/when we get it.
In my particular case, when I got it 2 times...I was able to discover that eating something with fairly strong garlic in it, is what caused mine. I've also talked to a few other people who've had the same discovery with garlic. So certain foods or spices may also be the cause of vapor's tongue for some people.
To get mine to clear up, I ate just 1 of those little Cinnamon Red Hot candies...the ones we used to buy in the little box at the neighborhood store when we were kids. Most convenience stores still carry them.