Vaping dehydrates you because the bases/carriers, both VG and PG, used in eliquids are humectants, moisture attractors. The majority of the vapor you see on an exhale is water drawn out of you.
Drinking lots of liquids, preferably water, is a long-term thing with vaping. It becomes unconscious over time, keeping liquids around. Fortunately for me I also really like flavored soda water, no calories or sweeteners.
On the plus side, like some others have mentioned I've cut down on my coffee consumption, essentially trading caffeine for nicotine (since I can vape in more places than I could smoke). I used to drink a lot of coffee a day at work, now it's one in the morning, one in the afternoon, most days, and it usually gets cold before it's done.