For anyone else, I looked at the source code myself and figured it out. You have to open the "BBAI_Game_Options_GlobalDefines" file contained within the XML folder of the Assets folder. Within that file, scroll down to the last define, which should look like this:
<Define>
<!-- Better BTS AI: Allows human player to offer themselves as vassal to an AI. The AI has been taught to value this appropriately. (Def: 0) -->
<DefineName>BBAI_HUMAN_AS_VASSAL_OPTION</DefineName>
<iDefineIntVal>0</iDefineIntVal>
</Define>
You need to change the <iDefineIntVal>0</iDefineIntVal> to <iDefineIntVal>1</iDefineIntVal> so that the end result is:
<Define>
<!-- Better BTS AI: Allows human player to offer themselves as vassal to an AI. The AI has been taught to value this appropriately. (Def: 0) -->
<DefineName>BBAI_HUMAN_AS_VASSAL_OPTION</DefineName>
<iDefineIntVal>1</iDefineIntVal>
</Define>
