I also don't like the vassal system. Firaxis created it in this way so that a master can't let a vassal become independent easily. Otherwise, you could give the vassal a few cities until he is just below the 50% mark and then give away one of your cities to someone else so that he goes over the 50% mark and then he becomes independent and you can declare war again. Not that this is that 'easy', but now it is virtually impossible to let a vassal become independent.
They didn't want to allow the vassal system to become an easy method to destroy a civilisation in two wars. I think they overdid it a bit as it is now not that interesting to get a vassal.
I don't know how to change it. You could ask for help in the
Creation and Customization forum where all the modders reside.
I will sometimes build the Spiral Minaret. Divine Right is more interesting if you are the first to research it as you would get the holy city of Islam which would allow you to get some additional holy city income if you have the production power to spread it through missionaries and can get a great prophet for the shrine.
The Spiral Minaret is more interesting in combination with the University of Sankore as both give bonuses to religious buildings. You can build temples that give +1 happy, +2 science and +2 gold and monasteries that give +10% science, +2 science and +2 gold. It also gives the gold and science bonus to cathedrals and the holy shrine of the religion.
If you have build a substantial amount of temples or monasteries in the past and want to continue to do that then the Spiral Minaret and the University of Sankore are interesting as they improve these buildings. If you wouldn't have build those buildings without one of these wonders, then they didn't suddenly become extremely interesting once you have one of these wonders. It just makes them slightly more interesting. If you have both wonders, then it would make temples and monasteries interesting even if you weren't planning to build them before.