Veng could not be possible with any recent rules sets. He is the result of I think the first spin of AD&D, just before Spelljammer came out.
My buddies and I, back in 1990, had a campaign going and we liked uber characters. We had a varient system of our own design hammered out for psionic points and mana points. Mechanically, it worked great, and we could do uber-things within the realms of the books we liked, i.e. Manual of the Planes, Deities and Demigods.
Veng was held in check mostly by TIME. It took uber-hours of questing to obtain those items - the building blocks needed to build a secret, mentally adjusted empire. It was all just getting going when, basically, our group disbanded and characters retired. That was 1992.
I haven't had an purely RPG session since - I went to PC games, and recently pretty much got suddenly bored with all of them! Whoa... And for the last year I have picked up Warhammer and am heavy into it.
Veng had no interest in taking over the world! He realized, in his world at least, that there were many deities being served by many factions everywhere. It seemed that deities had 'pet' countries or cities, and might intervene, or at least send an uber-champion, to deal with interference.
No. No nononononoooooo. Veng saw the True Path for him was to slither, un-noticed, into a position of total security. He loved his kind, in his own evil way, and took mates among them, and was content to satisfy their whims. He did not want to risk the comfort and safety of his enclave. They had it all now, thanks to him. Their main occupation was to make sport of the victims Veng would bring in through the mirror, and to make more Lamia Nobles. (Monster Manual, old one. They are dark elf on top, big snake below the waist. They have the natural ability to appear human at will. They have a no-save wisdom drain ability on touch.) If any of his enclave got out-of-line, he would shape-change, sneak into their dreams, and make some adjustments to their memories. He truly loved them and cared for them - ask any one them - they would say he was the greatest thing since sliced human!
So the True Path for Veng was in a different direction. He needed to research, research, research. That takes time, and he figured with all his alchemy extensions, he only had about another 60 years tops. His goal is secure a way to become immortal. There is some way for him to shed his corporeal shell and ascend into some kind of psionic demigod. And he will find it, and have fun along the way. Ultimately, he fancies himself as the God of Manipulation. Gods forbid!
Veng's most challenging adventures involve him and Spaz hunting down 11th level mages to be taken and wisdom drained and enslaved to work in his alchemy labs. For some reasons (I forget the exact details), 11th level was vital because the mages had no instant get-aways (usually) and they had access to a spell that could obtain information - a key and arcane spell that could give clues to obtain more artifacts and more clues to his ascention.
I can't recommend anyone try to incorporate Veng into a game but he sure makes for some interesting conversation! :)
For example, how do you REALLY know that Veng has not ALREADY taken over the world?!?!?! Muhuhahahahahaaaaa