Minimum number of coplanar vectors of unequal magnitude whose vector sum could be zero, is :

If there are two coplanar vectors of unequal magnitude then there resultant can never be zero.

If there are three coplanar vectors of unequal magnitude, then they may form a triangle. In this case , the resultant can be zero. 

So, we need atleast three coplanar vectors with unequal magnitude whose resultant (vector sum) can be zero.