Dragon Magazine #348 did an in-depth study on the secretive evil deity in a part of a series called "Core Beliefs: Vecna". A sample map of a temple of Vecna was artistically drawn up by Christopher West. Inspired by the entire article, this is the result - a temple of Vecna that can be placed in any campaign.
Cartouche heraldry by Anna Meyer, used with permission. Visit her page here: https://www.annabmeyer.com/
There is a DM, player, and clean version of this map on this post: https://www.patreon.com/posts/cults-and-evil-120905552