I'm not sure why DigitalPoint's Sphinx Plugin was designed this way. It is unrealistic to assume that all third-party mods will be able to include a dedicated Sphinx module (which is what DigitalPoint's mod currently requires). Instead, it should have been able to fall back to the regular search method if a Sphinx module was not available. But perhaps the way vBulletin's system was designed didn't make this too easy (possible issues with the Common-type search).
Unfortunately in order to fix this issue, I would need access to a Sphinx installation that uses Digital Point's mod. Sphinx is installed at a higher level than your forum installation, so this is not possible for many admins to give. If you can provide this, please fill out a Ticket Support service in the Members area, and give any additional instructions about how to access Sphinx on your server.