I still don't think it's much of a step above donor eggs, if Vash was fully apprised of how it was all going to work. It is true though that given Q's inability to spend 5 minutes mentoring a teenager Vash might be called upon to do stuff at some point. But I'm thinking she might find it quite convenient to have both a lover (or whatever) and a child who could potentially whisk her about the universe.
I think she would have tried to work out an angle to her benefit if he had made her the breeding proposal.