Let the four vertices be labeled A, B, C, and D. Since the labeling is arbitrary, there is no loss of generality by assuming that AB and BC are the given congruent sides. By definition of a parallelogram, CD has to be congruent to AB, therefore BC is congruent to CD also. Then for the same reason DA is congruent to CD which is congruent to BC which is congruent to AB. All 4 sides congruent = rhombus.
John
My calculator said it, I believe it, that settles it