Whup, nevermind. I found what should be the fix here: Photoupload works but wrong Thumbs ?!? _thumb.jpg
It seems that the code in vBGallery 2.5.1 is not following the documented syntax for substr since (reportedly) sometime in php4. With php5.3.1 php apparently expects the function to be called as documented or it no longer works.
Assuming this is accurate, will this error be fixed in 2.5.2?