First of all, that is not a problem. You can not use the "System - Page Cache" plugin on a VirtueMart site where the page contents are dynamic. The page cache plugin is designed for static content websites like blogs, business portals etc. You will face many problems using this plugin on a VirtueMart website.
Secondly, the code you changed utterly turns off all caching functionality of the plugin. Therefore, turning off the plugin would be more logical than deactivating it through codes.