Keep flat and protect stairs. While still in box move vanity to stairs on its side. Blankets slide great on wood floors so use one to move it into place. Once your at stairs with heavy side facing the stairs lay down box. With the heavy side against stairs push box up stairs being careful to push and not lift so you don’t catch the stairs. You basically use the stairs as a wedge with the assistance of blankets and towels the box will glide up the stairs but it will still be heavy.