Why V3+/V2+ reduction potential low. V2+ has half filled t2g so it should be more stable and value should be high

Dear student

First of all, I would recommend you to refer to the electronic configuration of both V²⁺ and V³⁺.
In the case of Vanadium 3+ the electronic configuration is 3d² and that of V²⁺ is 3d³
Now it depends what kind of ligand is going to come, and what kind of complex will be formed, for example if octahedral complex is to be formed then t2g will be half filled but if it is tetrahedral complex, then eg will be considered to be half filled. 
Also if we consider the maximum stable state of V, it is +5. Moreover the +5 has been reduced to +2, rather than stopping at +3. Thus, proving +2 is more stable than +3, and hence will be the values of reduction potential.

Regards