<p>What solutions do you have in place for handling bandwidth billing for your vhosts on a shared environment in apache?  If you are using log parsing, does your solution scale well when the logs become very very large?  Anyone using any sort of module out there for this?</p>

Although we use IIS rather than apache we do use log file analysis for bandwidth billing (and bandwidth profiling / analysis). We use a custom application to load data collected in the log files in one hour increments, and act upon any required notifications or bandwidth overuse.
The log file loader runs as a low priority process, so as not to interupt operation of the server. Even on high usage servers with a large number of sites, processing takes less than 15 minutes, so we don't see scalability as a problem with this methodology.
There may be better ways of doing this, but this is perfectly adequate for what we need. I look forward to viewing the other responses.