Use food grade PG or deionized water. You can get it at target or walgreens for $3/120 ml. Start with 5% and go from there as needed.
If you don’t have distilled or deionized water you can easily distill water yourself. Just take a heat proof container and sit it in a pot that is partially filled with water. Turn the burner on medium to low and put the lid on upside down. Put some ice cubes on the inverted lid. The steam will condense on the inverted lid and fall into the empty heat proof container. (I use a pyrex bowl) Just be careful, you don’t want to do a full boil on the water, and you don’t want to accidentally spill the melted ice into the bowl as you remove the lid.
Pure Grain Alcohol (Everclear -I’ve heard of using Vodka)
Ethyl Maltol — a sugar alcohol – but I’ve only ever used it already diluted in PG. It’s sweet.
Not the most recommended method but…plain old tap water. I have used it in all of my VG juices, DIY and otherwise. I have had no ill effects. Just add a few drops then shake well. If it is not his enough, throw a few more drops in. Not a big deal.
Overall, whatever works for you works. I wouldn’t go over 10% thinning material in juices however as you will lose vapor production.