I start with what would least contaminate the next, etc. so I go VG, PG, Nic, then flavors. If I went the other way, since I often times reuse syringes or pipette tips (changing tips only between flavors), I would contaminate my nic with flavors, or my base with nic and flavors. Going VG then PG, I help rinse the thick VG out for my PG based Nic.