However, with more factions, the option of surrender and becoming a vassal offers interesting options - with a view on backstabbing - for both sides. If you have six players (as an example) and one is losing and vassaled, but still in play, there might be interesting options:
1) The player you are a vassal of wins the game; in this case the vassal doesn't lose, but take part in the win; it becomes something like a forced alliance
2) The player you are a vassal of is beaten; in this case the vassal can simply operate normal again.
3) The player you are a vassal of is vassaled themselves; no status change for the first vassal (staying a vassal of a vassal)
4) A vassaled player is allowed to keep certain forces (X worth of units, like dwelllings or neutral towns in AoW3, depending on size). It can also operate secretly with obviously reduced capacity in order to secretly build troops over and above the allowed troops (but should the liege lord detect them it would give them reason to declare war again without consequence). Reduced capacity, because a vassaled player would have to pay taxes each turn to their liege lord, and because of the need to operate secretly.
5) Taxes might be taken in the form of freshly produced units as well;
6) At any time, if a vassal may deem it convenient, a vassal can rebel against their liege lord, With this action, the vassal forfeits the option to ever again in the game avoid defeat by offering vassalage - but might of course turn the tables on their erstwhile liege lord.
7) The liege lord of a vassal may demand to actively take part in any war the liege lord fights. The vassal must comply or rebel, but gets autonomous status as de-facto-ally.
8) At any time, the liege lord may declare war on a vassal. In case of a good reason (secret army), there are no consequences for the liege lord. In case of a brutal act of wanton aggression the liege lord forfeits the option of more players offering vassalage (every opponent will fight to the death).