You should look through your access_logs from around the time you started having problems and see if you can see what they did. You should also talk to your host about this. There is no date in the database for when a plugin was installed or changed. You may look the pluginid up in the database and then look through your Control Panel logs to see when that plugin was accessed/added/changed.
We recommend you follow the tips in this thread regarding securing your board -
https://www.vbulletin.com/forum/show...ms-More-Secure