I had to check this out. I found an old but presumably still accurate description from CFC
http://forums.civfanatics.com/showthread.php?t=141475
After some calculations and checking out the Civic upkeep costs I get that converting to Vassalage alongside with Pasifism costs me 0 coins
i.e. it is no-brainer.
Here are the calculations. Currently my N i.e total population is 72. I've 33 units of which 19 is military.
This gives:
Free support for units 0.24*72+7=24.28
Vassalage Free units 0.1*72+5 = 12.2
Pacifism free military units 0.12*72+2 = 10.64
Handicap cost = -9*0.2 = -1.8
(the last one seems to be suprisignly rounded down to -2)
On the other hand Vassalage costs in civic maintenance +7 and Pasifism is -16.
So this gives current units costs:
Unit costs: +9 (24 free units)
Military unit costs(Only pacifism): 0 (10 free units)
Handicap: -2
=> 7 coins
Unit costs in Pasifism+Vassalage:
Unit costs: 0 (24+12 free units)
Military unit costs(Only pacifism): 9 (10 free units)
Handicap: -2
=> 7 coins
So unit costs are exactly the same. Vassalage raises the civic upkeep by 7. However if I convert just to Pacifism I lose the Unit cost reduction so this is essentially even money proposition and I get the +2exp boost, if I want to finish some of those Longbows.
Overall I get quite big savings though since Pasifism is way cheaper civic than OR (-16). Net effect of this is +9 coins and probably also +1 coin from lower inflation.