Dr. Eli Vance was one of the scientists working at Black Mesa on the day of the infamous incident that led to the Combine's hostile takeover of earth. He was among the few that managed to survive, taking with him nothing but his infant daughter Alyx and a photo of them with his late wife. Refusing to let the aliens have their way with the planet and its people, he formed a resistance dedicated to driving them out.
A kind, grandfatherly old man, Eli will do whatever it takes to ensure the safety of those he cares about. He is also an inventor, and the creator of Alyx's robotic pet, Dog.
Dr. Vance's original voice actor (in Half-Life 2 and its two episodes) is the late great Robert Guillaume. This is just another of many great examples of the work of Robert's storied career, as well as in this franchise in general. His acting is subtle, but very emotionally effective. He really gives off a sense of the character's care for those around him, and has great chemistry with Merle that shines in their scenes together. Absolutely iconic performance.
Due to Robert's unfortunate passing prior to the production of Half-Life Alyx, the role was recast with James Moses Black. I have... mixed feelings about James' take. It's not that he sounds bad - his acting is solid, though to me not quite as deep or impactful as Robert's - but nothing about the voice makes me think of Robert's performance at all. It's so unlike Robert that I have a hard time believing they wanted a voice match. It's not that I don't understand taking the character in a new direction - as I said, no one would truly live up to Robert just by doing an impression. It's just that the game went to such lengths to recreate everything about its predecessors - including bringing back every living cast member - that hearing a character sound so different than they did before just makes them stick out that much more. That said, however, his performance is good when taken on its own; and it's a shame that it had to be so overshadowed by what came before.
At the end of the day, James turned in a good outing, but you can't beat the legend. I vote for Robert.