Water is generally used to thin an e-juice, particularly when using VG. I recommend 1 or 2 drops of water for each milliliter of VG used, but go easy at first, or your liquid will be too thin and that's difficult to fix. It's far easier to add a little more than to remove it.
When mixing, I highly recommend a viscosity test. Lay a mirror down flat, and place a drop of your liquid onto it, and then a drop of another liquid that you know vapes and wicks well. Lift the mirror to 90 degrees, and see which drop flows faster.
If your e-juice flows faster, then it's thinner and you might want to consider thickening it with more VG. If the other e-juice flows faster, then you should thin yours with a little water, shake well, and try again.
Or just vape the stuff, and see if you notice a difference in vaping and wicking properties. The viscosity doesn't have to be perfect.
Eventually you will get a general idea of how much water you like in your recipes.
Vodka can be substituted for water if you wish to add a little more throat hit.
If you use 1 measuring device for an entire session, rinse it - inside and out - between ingredients. Put a cup of very hot water at your mixing work area, and use it to rinse the syringe/etc.
Using vodka as a final wash is an excellent idea.
