well, to be 100% vg, everything you put in there needs to be vg-based already
i.e. vg nic, vg base, and vg flavourings.
most likely you can find vg nic and pure vg, but vg flavourings are a bit scarcer.
so you can end up with (hat tip to Tesco's burgers) "everything vg based in my liquid is 100% vg"
vg is also very very gloopy. Will be a pain to topup tanks and whatnot
we can do some rough examples, if you like
e.g. your nic is vg based and 70mg....you base is vg and you flavouring is pg
a 30ml bottle mixed at max vg, 12mg~ , 15% flavouring will be roughly
~5mls nic (vg based)
~5mls flavouring (pg based)
~20mls vg (obviously, vg based)
(ignoring decimals to get a ballpark figure of ratios)
so your mix will be 25ml's of stuff that is vg based, and 5mls of stuff that is pg based.
which is about 85% vg (rough maths folks...:-) again, for ballpark figures)
the above "recipe" is what I use when I'm not too worried, it's easy to remember, easy to measure