No, VIP to premium would not open all created characters to 20. It would allow you to level them as far as you want/can while VIP with no sigils. Once going to premium, you would then be restricted by whatever cap you are under.
I.E. You go VIP and create 2 new characters. 1 you level to level 7 in a month and then go premium. You did not need a copper sigil to advance beyond 4. However, to go past level 8, you will now need a silver sigil. The other character you get to level 9 without sigils. He will be able to advance to 12 without any issue. If you want to go beyond, you'll need the next sigil.