Wouldn't you transfer Aristo's requirement of Caste System onto Vassalage? Then it doesn't have only one prereq.
I was going to put Caste System as a prereq for Monarchy, and Iron Working for Vassalage. Caste System to Vassalage is a bit too long a jump for my taste.
So go for it.

the higher it is like an evil twin of the culture slider. I know some civics have this already but you could remove those to streamline them. you could also introduce some new buildings like Tax office which could give a gold bonus but cause additional
from the culture slider. Other ideas for it which I don't know if they are possible would be having certain civics like say free market have a hard limit on how high you can go on the tax slider. Or unlocking only parts of the tax slider at a time like say at trade you can have the tax slider at a maximum of 10% and then once you get currency you can have it at a maximum of 20% and so on.