There is room to vassalize 2 opponents that don't want to change of their own free will
On the contrary, there is room to vassalize the entire world minus one civ (to avoid conquest victory). As long as THEY declare war on YOU, there is no negative modifier for "you declared war on us".... and you are free to prosecute the war as you see fit (except that eliminating the civ would put your team out of the game). I think.
Is this indeed a correct interpretation of the requirement for the diplo modifiers?