Wikipedia lists this background:

In Eldritch Wizardry (1976), the third supplement to the original Dungeons & Dragons rules, Brian Blume invented two artifacts he called the Hand and Eye of Vecna. These were supposedly the only remnants of an evil lich, Vecna, who had been destroyed long ago. "The book contains no more detail on Vecna other than the fact that Kas was his bodyguard". The name Vecna was an anagram of Vance, the surname of Jack Vance, the fantasy author whose works inspired the magic system used in Dungeons & Dragons. The Hand and Eye of Vecna on the other hand were inspired by similar items that appear in the Eternal Champion series by Michael Moorcock (the Hand of Kwll and the Eye of Rhynn).

On the artifacts, Gary Gygax later said, "nary a detail of those items did [Blume] ever reveal to me".

Then,

Ten years later, in Advanced Dungeons & Dragons 2nd edition's Dungeon Master's Guide (1989), Vecna's history was expanded under the description of his Hand and Eye. With the release of the adventure Vecna Lives! in 1990, written in support of The City of Greyhawk boxed set, Vecna finally appeared in person, re-imagined as a demigod, and the chief antagonist of the adventure.