Vbulletin should not archive old post unless you set it up to do so. And what do you mean by the "archive"?? There is a search engine friendly archive. http://www.vbseo.com/f34/how-completely-turn-off-vbulletin-archive-24545/
Do you have pruning enabled?? Vbulletin control panel - Threads and post - Prune. Check the settings inside of there. All of those boxes should be 0 or -1