That s not complete code. It does not contain variable declarations. Your first post stated that tau and b are variables, although it did not explicitly state that they are the only variables. Your code looks suspiciously like vn may also be a variable.
In any event, you can’t expect anyone to help you unless they clearly know what the optimization problem is, to include knowing what the optimization variables are, and what is input data.
If vn is an optimization variable (and if it is not, why are you applying CVX functions to it?), the problem doesn’t look convex to me.