vpasolve was introduced with R2012b. If you have only R2012a, then you cannot use vpasolve.
If a product was not written for the release that you have, you cannot just add it. You cannot use a newer toolbox in an old release. Sorry, but that is not possible.
If you really feel you need that product, then you need to upgrade your MATLAB release.
The alternative is to find another tool that provides similar functionality. For example, you may be able to use a tool like fsolve, from the optimization toolbox.
Sorry, but your choices are severely restricted if you insist on staying with an old release.
If you actually have R2012b, then you need to buy the symbolic toolbox, for your existing release. For this, you will best contact customer support.
