We have this condition in all 3 of our installers:
<Condition Message="Visual Studio 2013 must be installed. The free integrated shell (and its dependent isolated shell) can be downloaded at TODO."> VSINSTALLPATH OR Installed </Condition>
For the other VS versions, we have the URL to the download. We'll either have to rephrase so that we don't put the link there, or manage to get the link to RTM download and rebuild PTVS with the final link.