i think vincent is bi with a preference for men, and part of the reason why he really leaned into infatuation with lucrecia was because she was the first woman he'd been into in a while and he had a whole comphet style"oh thank fuck i can be normal" complex about it.
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the other half is that he had extreme trauma happen to him directly following and directly because of his crush on her and so he latched onto the affection he felt as the last positive emotion he experienced for like. 3 decades. if he'd had a normal life after niebleheim he probably would have gotten over her like a normal person
i really should finish dirge because atm i have no idea if she reciprocated at all or if this was totally one sided and couldn't really pick it up anything i could trust from just reading the script