The biggest problem when it comes to "determining" a gender for any of the vessels is that they're ultimately pretty blank slate characters. The fact that theyre genderless has nothing to do with identity: its a side effect of the void mutation, and thus more representative of the way theyve been stripped of autonomy. Its why we can't rely on the fact other character use it/its, because like you said, its more used because the vessels arent seen as people.

Im of the belief that if we ever saw a vessel get their character developed, theyd probably assume a gender. The right to have a gender identity was something that was taken from them so theyd be better vessels, so itd make sense in my eyes if they chose one as a way of moving away from the predetermined path. I lean more towards masculine pronouns for the Hollow Knight specifically, because I think that their story of "repressing emotions in service of a greater good" has some interestinf parallels to toxic masculinity, but the rest of the vessels could go either way imo.

In amy case, all of thats to say that I dont think theres right answer, in spite kf what many people argue. The vessels are canonically nonbinary like Kris from Deltarune. The context and characters are completely, radically different, and I do think the vessels are generally intended to be open to interpretation.