PG (Propylene Glycol) and VG (Vegetable Gycerin) are both bases for juice.

PG has a thinner consistency than VG.

A higher percantage of VG in a juice provides more vapor however can mute the flavour.

A base standard is generally 80% PG and 20% VG to give maximum flavour and vapor. From there you can customise to personal preference or if there are any allergies to either.