As
@thetopher pointed out, William knew more about what was happening to him than Liam did. Darla made sure that Liam didn't see her face so he wasn't completely aware of what was happening. She kept something from him, which meant that Liam wasn't aware of everything that was happening and wasn't in a position to actually give any sort of consent. Drusilla made sure that William saw her full face, made sure he knew that there was something more going on that what she was saying, but William didn't stop her to ask any questions, he let her proceed.
William was in a better position to consent, even if I still think that neither of them consented to be a vampire.
As to the consent thing, it wouldn't matter what any man - or woman - said to me. If I didn't have all the details, if I didn't know 100% what was happening or going to happen, I would never say 'yes'. I wouldn't be able to. It's why people are fighting so hard for the rape laws to be changed.