Well if you think about it it doesn't get more lush and full of life than a marsh.
A marsh is a feature on top of a grassland IIRC
Trying to vitalize one would be like trying to vitalize jungle
No love for grassland floodplains? One of them can feed 8 people without a grain resource. And with slums you can have many. It just takes enormous luck in map creation or a premade scenario. Imagine all that food.
1 farmer feeding 7 priests is bad you think? With altar 3, scholarship, caste system and the hall of kings, these guys will do everything for you. An optimal location get you 160 food for a normal city, 288 for the slums city. A vampire each turn and it gets very scary after just one feast.
You could almost literally build vampire lords. (needs lvl 12)