I have hard wood floors everywhere except the kitchen and bathrooms. The ONLY reason those floors are not wood is because I worried about water damage. If your vinyl planking can take the water, put it all over.
Consistent floors make the house look bigger. It is easier to clean if you can use the same products and tools on all floors.
Tile grout is a pain to clean anyway!
Under no circumstances should there be off white carpet in any room unless he intends to clean it and realizes it will need replaced more often than your planks.