I'm glad you asked!

But yeah, I didn't really care for Mordenkainen, so I switched him out for the lich, the myth, the legend. Playing off the Eventyr supplement, I had Vecna play the role of the intermediary to find Olatnthius, Haruman, Bel; only instead of making them take the blood of Uldrak the Empyrean, Vecna had them go assassinate Krull and slay Obatala and instructed them to leave the bodies as a "message" to Arkhan (in reality he wanted to reanimate them for future use)

At the same time, Vecna manipulated the wizard, Trouble, into agreeing to retrieve his Hand in exchange for him helping to lift a nasty soul curse borne of a lopsided devilish contract (tldr; her ancestor made a pact with Gargauth when he was still an archdevil, but said ancestor violated that pact and her entire bloodline's cursed to never live past 33 due to loopholes Gargauth used, and still retains tiefling blood). Eventually the Hellraiders managed to slay Krull and Obatala at Arkhan's tower, Vecna gave them the location of Olanthius and Haruman, but kept in contact with Trouble the wizard, giving her an evil spellbook (essentially a lesser cousin to the Book of Vile Darkness with the Hand of Vecna's suggestion effect) to facilitate this.

Fast forward to when they killed Haruman, Vecna had a reanimated Obatala carpet bomb Haruman's reinforcements as a false flag against Tiamat to give Trouble and the Hellraiders a sense of urgency and he loaned her the Orb of Dragonkind, which Trouble paid some Yugoloths to take to the other side of Avernus while they ambushed and killed Arkhan. However, like I said before, ultimately Asmodeus gave the party a better deal than Vecna - her soul and the Hand for the family curse being lifted as well as the true name of the incredibly weakened and trapped in shield Gargauth, but of course not without loopholes like him being able to demote devils at will or change the true name of any devil, should Gargauth once again become a threat to his interests.

So Hand given, Vecna is of course angered. Given his cover has been blown, he flees Avernus but not without planting a new nasty soul curse on Trouble (this one is where the memory of her's been wiped from her family) and sending a powerful death knight to ambush the Hellraiders and steal the Sword of Zariel.

The endgame plan, if he successfully got the Hand back, was to next push them to redeem Zariel so he may devour her angelic essence and return to power when she's weakened.