I know you've already bought DW, but thought I'd chime in. I use VG based nic, VG, and flavoring. I don't necessarily think you
need to thin it at all, unless you chain
vape, or you're chucking clouds. But it really depends on your style and equipment. I'd try it without thinning first, to see if even you need to.
And even then, I'd experiment with R.O. (
Regular
Old) water. I wouldn't worry about impurities. I know, someone may mention potential volatile organic compounds. But- I mean, how much vapor from that Regular Old water do people inhale in the shower every day? Water boils at 212F, and you'll be
vaping at 400-450F, so it's
way past boiling, and "cleanliness" isn't an issue. So really, the biggest concern would be TDS (total dissolved solids) and minerals that could crud up your coil. When first starting diy, I tried thinning just with tap water to see if I liked it...and found I had no issues with all vg without thinning (which makes it that much easier).
But really, if you
do need to thin for your
vaping style and gear, long term, distilled water would likely have the lowest effect on your coil, and might give more peace of mind. Just my $0.02.