You need 3 vassals minimum.
Each with a different specialisation and each to max. level.
I won't go into detail how the vassal system works, but here's some tips:
Apparently, the amount of taxes you receive or pay, depends a lot on the size of your empire but also on the vassal empire. This is important if you struggle to maintain the high cost of a Bulwark vassal, just do it on a real tiny empire (or vassalize one of your smallest districts) and the cost won't be so high.
Obviously it's easier to force smaller empires to your will, thx to power difference, but that's up to you.
What I found is to best do it in certain order:
1) Scholarium: by far, the easiest to maintain. get it first. any size vassal will do.
Once the Scholarium is lvl2, get:
2) Prospectorium: it won't be easy to convince any vassal to become a Prospectorium. If youy're lucky, you might get one "for free" if you vassalized an overlord. If not, just be patient and modify their contract a little bit every 5 years into real crappy conditions, so when you offer to make them Prospectorium they'll accept
Once the Prospectorium is lvl2, get:
3) Bulwark: everyone wants to be a Bulwark. "it's raining free resources, yo!" but for the Overlord (you) it's extremely hard to pay for, specially when they level up. So my suggestion is to get a tiny empire/vassal to commit to this role and it will be real easy to pay the resource tax.
I hope this helps.
chievo should pop once the 3rd specialised vassal levels up to 3