If you use a high-glycerine mix then starting with VG-nic base is a good choice.
Depending on the vendor, glycerine base may be ready to use, or it may require dilution with DW (distilled water) or any other less viscous liquid such as PG or alcohol. For example Ecopure Krystal VG base (@36mg) is already diluted with 20% DW and ready to use; RTS Vapes VG base (any strength) is too thick to use as-is and requires a minimum of 10% dilution (20% is better).
PG is a good diluent for thick, goopy VG and a 70-30 VG-PG mix will work just fine. Actually it won't be 70-30 in the final mix because there will normally be 10% flavour or more, and flavourings are usually alcohol or PG based, thus increasing the dilution. So a final mix that consisted of 70-30 base will be about 65-25-10 VG-PG-flavour, and a final mix that has 65% content of originally highly viscous glycerine base will be absolutely fine; it will be slightly more viscous than 70-30 PG-VG but only on close inspection.