A special case for the subset sum problem
Obviously, even numbers cannot add up to an odd $M$. Other than that, we could just set $v_i = w_i /2$ and $N = M/2$. Then finding a subset of $w_1, \dots w_n$ which adds up to $M$ would be the same as finding a subset of $v_1, \dots, v_n$ which adds up to $N$, where the latter is the unconstrained knapsack problem.