This is due to two bugs, actually.
I've uploaded a fix as well as a backwards-compatible workaround, the affected files are:
/dbtech/vbshout/includes/libs/shoutbox.php
/dbtech/vbshout_pro/includes/hooks/dbtech_vbshout_post_save_once.php
When this is applied, it will also fix the bug where shouts made in an AdminCP-created chat room that spans multiple instances will not show up in the other instances from the one where the shout was created.