Flooring multiple rooms
Generally even luxury vinyl plank has expansion requirements and typically sets maximum room widths.  You'd probably end up with a T molding separating the rooms.  Now if you are asking about a reducing molding that steps down to a different floor height then I'd try to avoid that.  

Check the instructions on your specific product to see what maximum they specify sometimes you can go over the maximum by increasing the room left for expansion in which case maybe you can avoid a molding entirely which is going to be the best look.

What installation type are you planning? floating or glue down?