Good point about the Kishoumon, this was effectively quite similar to the oath sworn by western vassals. The difference is the lack of a legal framework, which is what I was referring to. The oaths of samurai were more familial and religious in nature, based on custom rather than institutions. Here's a couple of extracts from 'Japanese Civilization: A Comparative View' by S. N. Eisenstadt, which I used as a source:
"In Japan relations between vassal and lord were generally couched, not in contractual terms based on fully formalized mutual legal rights and obligations, but in terms of familial or filial obligations. Within this structure vassals exercised no principled legal rights vis-à-vis their lords..."
"This does not mean, of course, that in Japan there were no de facto modes of consultation among vassals and between vassals and their lords. But such consultations were ad hoc, structured according to situational exigencies and custom, not according to any conception of inherent rights of vassals either individually or as a body"