You can use water, vodka, or even everclear to thin juices if you need to. Alcohol will add some bite and throat hit (the bite is not always a welcome sensation.....). But it takes very little to thin out a mixture enough to get it ti vape better. If you go more than 5-10% that is too much; you will most likely kill the flavor. You want to avoid adding any alcohol that has sugar in it (like Rum, but particularly cordials. They seem like a good idea, but aren't!!!).
Having said that, now I'm going to have to try a little single-malt in a vape (perhaps an RY4).
Sorry, I digress. If your liquid is too thick, one thing that often helps is to just take a couple of dry pulls (breathe in on the e-cig without pressing the button). This can pull in more liquid and get the wick/coil better moistened. Or try a different type of head.