Different antivirus scanners have different success rates, and operate in different ways. Some antivirus scanners scan every file as it's downloaded or saved--these slow your computer down a LOT. Some scan files only when accessed.
It sounds like VB scans when files are accessed. ClamX accessed the files in your mail downloads folder, so that triggered VB to scan them.
Given the relative scarcity of Mac malware, running two antivirus programs seems like overkill. It will have a significant impact on your computer's performance.
