well here is a few checks u can do if u are on a shared hosting.
Connect using ftp , check for any files mostly starting with a "." , a common practice for hackers to upload a shell script and name it starting with a dot which makes it invisible in some searches.
In your VB forum folders, check if u have left "IMPEX" folders "INSTALL" folders, if u did delete them ( this wont stop them along) but these folders can easily be used to upload a shell script or other exploit.
Another trick is that , the shell script access, is still to be recorded in ur weblogs, check that carefully, i have found many info in the past reading the accessed urls and which file was used to upload and which file to exploit.
There are also more checks, i have listed the easiest u can do with minimal access to the server itself.