I use distilled water to thin my VG DIYs. Anywhere from 5 - 20%, depending on the amount of flavoring I use, and how much that flavoring thins the juice itself. General rule of thumb for me is flavor% + water% should be about 20%, if the flavoring is PG based, as they generally are. I have had some unthinned premade juices that were VG based. Very thick, but also very nice for dripping. Cartomizers need thinner juice to wick well in the filler.
Not all high-nic VG unflavored juices are the same. MyFreedomSmokes 100% VG really is very thick, and Chris has told me it is nic + VG only, no thinning. Vermont Vapor 35 mg VG does have water in it already, so I use less water to further thin it for a DIY, and sometimes none.
I am also someone that does NOT like alcohol in my juices. I did use it for a while at about 10% to thin my DIYs, but they always dried me out too much, much like the reason I do not use PG, and they actually made me fairly light headed. Inhaled alcohol vapor does indeed go right to the lungs and the brain. Anyone that has used a brandy pipe knows that one can get fairly high on only a couple mL of inhaled alcohol. This is not a judgment call on the vets that do use it...it is a very common thinning agent that the majority here do not have problems with...but I do not drink alcohol at all, and so any effect, especially with my scientific research or martial arts time, is quite noticed. And I was trying to avoid the dryness that PG produced in the mouth and throat by going to VG juices, and alcohol was doing almost the same.
Just my $0.02.