[kmike]

The large traffic between the db backend and web frontend is not unusual for vBulletin - apart from the page data, it retrieves a datastore on each request, and some datastore entries are unnecessarily large and/or not needed on each page. So if the forum activity is high enough, 40GB/day is nothing unusual. One of our forums is generating an average 60GB/day backend traffic (and that's with datastore caching enabled and vB Optimise installed), another fairly active forum is close to 350GB/day on the peak days.

What's your board activity? How many forums do you have?
Could you run this query, adjusting YOUR_FORUM_DB accordingly:

SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE table_schema='YOUR_FORUM_DB' and TABLE_NAME='datastore'\G

You can reduce the backend traffic by enabling one of the local datastore caching methods in vBulletin: XCache, APC, eAccelerator. You can also use memcached, but it has to be running on the same server for the traffic to stay local. You can further reduce the backend traffic by installing the vB Optimise product.