I have worked out the solution to this problem, and hopefully it will help others.
First consider an (arbitrary) inequality
x - y < 5
we can rearrange this to be an inequality with some algebraic tricks, consider this rearranged to
x - y - 5 < 0
then if the inequality holds true the left hand side must be negative, this can be made an inequality by use of the sign function:
sign(x-y-5)==-1
now, MATLAB does not allow more constraints than parameters, so we just add a fake parameter!
For example, the solution to the previous code would be:
syms x dummy
[xx,~]=vpasolve([x^2-12.5*x+37==0,sign(x-5)==-1],[x,dummy])
where the tilde suppresses the output of the unneeded dummy variable. Now if the answer is less than five the equality is held.
Cheers,
Jesse