Can Vayne return to hand while chained to Sacrifice during a conquer?

That line does not work. You do not lose Vayne's Assault bonus until the end of combat, which includes the conquer step, so she can be 5 might when conquering — but if she is dead she cannot return to hand, and if she returns to hand she is not sacrificed. Rule 359.3.e.2 prevents the return if she has been killed.