Vida Sherwin is probably the closest thing Carol Kennicott has to a best friend in Gopher Prairie. For one thing, Vida agrees with Carol that the town is boring and badly in need of improvement. Better yet, Vida has a lot of energy that Carol is afraid she herself will lose in Gopher Prairie: "[You] never did look at Vida Sherwin in detail," we're told. "You couldn't. Her electric activity veiled her. She was as energetic as a chipmunk" (5.5.3).
When Carol first meets Vida, she feels like she finally has someone she can talk to. Unfortunately, even Vida has a way of making Carol feel ashamed of her constant dissatisfaction. In fact, as the novel unfolds, Vida takes on a sort of big-sister role for Carol. Yes, Vida has many of the same thoughts Carol does—but Vida is judgmental about some of the ways Carol tries to accomplish her plans.
Eventually, we find that "Carol had avoided exposing her plans to Vida Sherwin. She was shy of the big-sister manner; Vida would either laugh at her or snatch the idea and change it to suit herself" (11.5.1). In other words, Vida is sympathetic but also overbearing. Like Carol, she likes to do things her way.
What Carol doesn't realize until the end of the book is that Vida actually has a good reason to resent her. You see, Vida was a little romantically involved with Will Kennicott before Carol came into the picture. As you can imagine, it can't be easy for Vida to stand by and watch while Carol makes Will's life difficult; Vida can only think about how much happier Will would have been if he had chosen her instead of Carol. At one point, she even thinks: "Men are such fools. I'd be ten times as sweet to make love to as that dreamy baby. And I am as good-looking!" (21.1.26).
Here, we get a glimpse of the pain that Vida feels deep down, and that gives us some insight into the people of Gopher Prairie, who can't possibly be as simple and boring as Carol Kennicott thinks they are. The problem is that Carol can't accept the fact that other people are just as complicated as she is.