You've pinpointed the correct story in the comments, so if you want to add your own answer I'll delete this. For now I'll answer in case anyone wants more information.
The story is called "Research Alpha" and was cowritten by Van Vogt and James H. Schmitz. It appeared in the anthology More than Superhuman, and also also appeared as a subplot in the novel Supermind. Van Vogt had a tendency to bolt together his short stories to make novels - not always with happy results.
Spoilers follow:
The underlying idea is that the Great Galactics have injected some of their own genes into the races they encounter, but during human evolution these genes have become unevenly distributed. The scientist Dr Gloge is working on a project called Point Omega Stimulation which aims to enhance human evolution. Gloge secretly injects two people, Barbara Ellington and Vincent Strather with the Omega serum. Barbara possesses an unusually favourable package of Great Galactic genes and the serum enhances these so she basically turns into a Great Galactic. In Vincent the serum enhances other genes and he becomes a goblin like figure - it's implied but not stated explicitly that he's become mentally subnormal.
In Supermind it's made clear that this is a deliberate manipulation by the Great Galactic William Leigh. He had detected the favourable genes in Barbara and wanted her to attain her full potential so he can have her as a companion. Vincent was included in the experiment to demonstrate to mankind that the process would mostly fail so it shouldn't be used routinely. As far as I recall this aspect of the plot is absent from the original short story.
Vincent recovers, so it's a happy ending :-)
