jmc

Since $v$ appears in the first row, first column, and on diagonal, the sum of the remaining two numbers in each of these lines must be the same. Thus, $$25+18=24+w=21+x,$$ so $w=19$ and $x=22$. now 25,22, and 19 form a diagonal with a sum of 66, so we can find $v=23$, $y=26$, and $z=20$. Hence $y+z=\boxed{46}$.