The original actors were reprising their movie roles, much as they had in various media throughout the last couple of decades (this would be the final Ghostbusters appearance for Harold Ramis, who'd pass away in 2014). As for von Sydow, he was doing the voice of Vigo, who'd appeared in Ghostbusters II. They had a pretty good reason for not bringing in Wilhelm von Homburg, who'd played Vigo on the big screen. "Unfortunately, that actor had passed away," explained the game's director pre-release, "so we'll have a sound-alike."
For a second-choice understudy, von Sydow was a pretty big name. This was Max von Sydow of The Seventh Seal, Max von Sydow of The Exorcist. More recently, you might know von Sydow, who died this past March at age 90, for Game Of Thrones and The Force Awakens. Players then noticed that calling him a "sound-alike" was kind of an understatement. Vigo, in the game, sounded exactly like Vigo in the movie, even when delivering new lines. So if von Sydow was voicing game Vigo, what did that say about original Vigo?
Fans badgered producers and uncovered the truth: Von Sydow had voiced the original Vigo after all. Von Homburg played the role and did the voice originally, but then the production brought in the big guns and had von Sydow dub over them. Von Sydow wasn't credited, and even von Homburg had no idea his voice work was being dubbed over until he attended the premiere and heard the final cut. He ran out, furious. Even many who worked on the game didn't know why von Sydow was brought on board. Hell, maybe even von Sydow forgot the role was his. When you have 163 acting credits, they tend to run together after a while.