Make a saturated solution of Stevia in 20ml of PG. By saturated I mean dump say a tsp. in and shake it up real good then put it in some hot water for a while. When the stuff that won't dissolve settles, decant off the remaining clear fluid. No idea what the percent of Stevia is but try using it at 3 drops for every 10ml of juice (the glass droppers that come in medicine bottles for example -- prettty large drops) .
So far it seems to work well and gives the juice a truly sweet flavor. Also doesn't gunk up your coils like other sweeteners... YMMV
umm this is liquid.. so do a 10ml bottle.. add 9ml of pg and 1 ml stevia..
that should help a bit better.. increase as needed.. (but really do not need much sweetener..)