About orientation

I would like to know the simplest way to define the orientation of an object, given that I know its forward direction being VectorA, and its up direction being VectorB, where VectorA and VectorB are perpendicular to each other.

I have tried

 transform.forward = VectorA;
 transform.up = VectorB;
 

But one will overwrite the other so I cannot get the perfect orientation. Thanks for help.