V.E.T.O. means "Vetoed from Exposing, Touching or Obstructing" (or "Vietato Esporsi, Toccare, Ostacolare" in Italian), and is establish to help Angels and Devils to guide or corrupt mortals without depriving them of freewill. Young Angels and Devils who are learning to become Guardian Angels or Guardian Devils are paired-up with an unknowing "Earthly one." They have to take turns to influence their Earthly one, while taking a human disguise to guide them in the right, in the Angels case, or wrong, in the Devils case, path. As this is happening during the Earthly Ones' formative (teen) years, their subtle influence would help shape their moral and ethical development into adulthood.
This rule also keeps Angels and Devils from having intimate contact with mortals or each other. When Raf touched Sulfus' hand, they broke the V.E.T.O., and they got a star shaped mark on the hands that had touched the other.
However Raf and Sulfus were not the first ones to committ a sacrilege, the very first one was committed by Tyco and Sai, a rival couple that was in love with each other.
Reina also committed a sacrilege, but not like the one Tyco and Sai committed, she too was in love with someone she shouldn't, her Earthly One, Malachia, and in order to make him fall in love with her, she stole his portrait which deprived him of his free will.
In order to keep the balance of the Universe and also to stop a war between Angels and Devils from starting, the High and Low Spheres created the Temple of V.E.T.O where there lays the Libra of V.E.T.O, if destroyed then the Eternal Law that keeps Angels and Devils from touching disappears.