Anything which goes into a CVX expression must be a MATLAB double, not something such as VPA. sym is not allowed in CVX. CVX and the solvers it calls use double precision, and can not do 32 digit calculations.
Using VPA for some preliminary calculations is o.k. as long as whatever is provided to CVX has been converted to a double before doing so. If you have issues with converting vpa to double, I suggest you seek help at https://www.mathworks.com/matlabcentral/answers/index .