which is more stable V³⁺ or V²⁺ & why?

Please note that the most stable oxidation state for vanadium is neither +2 or +3, but +4. The +5 oxidation state is slightly oxidising while the +2 and +3 oxidation states are reducing. Out of +2 and +3, the +2 oxidation state is more stable. Both +5 and +3 oxidation states are reduced to +2, suggesting that V²⁺ ion is the more stable than V³⁺ ion.