I think both Hoyt and Citra are equally to blame for how Vaas ultimately turned out.

Hoyt was a much worse influence. While Citra was no better, she was following her people's beliefs and traditions as the Rakyat Tribe believe in a warrior like culture. Hoyt is a cruel and vile crime lord who exploited Vaas's troubled past and offered him money, drugs and power to recruit him just because he wanted Vaas around as a violent enforcer to maintain his control over the Rook Islands and has encouraged Vaas to be as cruel and sadistic to the locals and anyone who opposes Volker's criminal empire.