Each extra culture level past Legendary 1 should give an additional VP. A Legendary city is 4 VPs, getting to Legendary II should make it 5 VPs. Is that not the case?
What I meant was to increase the scale. Legendary I -> II = +2 VP (up from +1). Legendary II -> III = +3 VP (up from +1).
The basics of weak to legendary are fine, I think the VP amounts are perfectly fine for the effort it takes to grow that culture. Its legendary II on....its takes so much work to hit those numbers, that I think VP should be more heavily weighted there. This allows for a civ who is playing "tall" to have a better chance at VP....they aren't conquering cities, but my god once a few legendary IIs come online the VP really starts growing.... that would be the idea.