First, the "unit vector deal" you're having problems with is just notation; in your example, x is he same as [3, -4, -1].
You're not quite right about hoping they have equal magnitutes. A parallelogram with equal-lengthed diagonals is a rectangle, not a rhombus. A rhombus has perpendicular diagonals, so they want you to take the dot product and show that it equals zero.
Speaking of which, I'm guessing you copied the problem down wrong: either x=3i-4j+k, or y=[3,2,+1].
Hope this helps.