Vecna Lives is pretty widely considered to be terrible. This review sums up its flaws, nicely, but the fact that it requires all the gods to be idiots and the players RP the Circle of Eight as complete boobs in order to get wiped out. How exactly did they reach such high levels when their plan of investigation consists of "run into an unknown tomb with no preparation".
For all its bright spots, 5th edition is pretty terrible at "party vs one thing" and really should have stolen more from 4E in terms of "simulating stories" of heroes vs. an epic evil. I guess give Vecna the lair power of "Gains 2000 temporary HP" or something, because 8 wizards vs anything is going to be a joke, as he'll be focus fired to death the first round. I'd personally skip the terrible intro and just flash to the actual PC's catching glimpses of their mentor's deaths, and run it with them around 7th level. Knowing the Circle was killed by the thing you now have to go against should be scary enough without them having to sit through the cheating and railroading required to kill them in game time.
I am reading the adventure and, even considering some metagaming and plot devices that surely are a reminder of the late 80's adventures, I am enjoying the plot and the adventure itself! I was thinking exactly about the points that you and the review addressed, and I believe that including some backstory related to Vecna’s preparation, like the preparation of a ritual created by him that obscured the knowledge of his plans, explaining the reasons for the gods and the archmages of the Circle of Eight are unaware of his plans, could explain a lot of issues. He is (or shall become) the god of secrets, after all!
In addition, in the initial encounter, I included in the map two tombs for creatures that were serving Halmadar/Vecna (in fact, they were serving the artifacts, the Hand and the Eye of Vecna), that were imprisoned along with him. They are exactly the Eye and the Hand, the golem-like creatures that could not be destroyed, and were in turn buried along with Halmadar/Vecna.
This could help me with the problem mentioned by you of the “party vs one enemy” thing. My first idea would be to have Halmadar (in fact, a kind of Apect of Vecna dominated by the artifacts), along with the Eye and the Hand in combat. My second idea was to allow the Aspect of Vecna summon a very powerful creature (a Balor, for example), to fight alongside with him against the Circle of Eight - Vecna made a pact with Orcus, and this "help" could justify the summoning of demons in the combat. Additionaly, since Halmadar´s Tomb had several wards and protections, not every spells of the characters would work properly, indicating that the combat would be more hard than initially expected. In this repect, and considering the experience of forum members regarding Fifth Edition here, how would you create the encounter that could wipe out six players (two members shall be NPCs) using high level wizards in D&D 5e?
I particulary like the introduction, because it can provide the players the reasons to fear and to be aware of Vecna’s power. In addition, it would be interesting (and fun!) to see the players surprised due to the deaths of all members of the Circle of Eight.
I shall check Return of the White Plume Mountain as an option as well. I shall also adapt in the future Expedition to Castle Ravenloft and Expedition to the Ruins of Castle Greyhawk.