[final kaoss]

You'll be just fine using mod_deflate. Just turn off vbulletin's gzip function otherwise, you'll be increasing load for no reason plus I don't believe vbulletins gzip feature covers everything.

The mod_deflate module is similar to mod_gzip, but usable only with Apache v2. Early versions of mod_deflate provided lesser amount of compression than mod_gzip.[2] Starting with Apache 2.0.45, the compression level of mod_deflate is configurable using the DeflateCompressionLevel directive, so this difference disappeared.
A mod_gz module was independently developed by Ian Holsman. This module implements a gzip compression filter for Apache 2.0, providing similar functionality to mod_gzip. One important difference between the two modules is that mod_gzip includes its own gzip implementation, whereas mod_gz relies on an external zlib library.