I think there is indeed symbolism in Vronsky's bald spot. One of Karenin's chief characteristics is his ears, which is often the first thing people notice of him. Ears are meant for hearing—hearing rumors, gossips, scandalous affairs. Whereas a bald spot is something that people often conceal. It could easily symbolize an adulterous relationship. Consider this scene at the military camp, in which an officer calls to Vronsky as he leaves his Finnish hut.
"Vronsky!" shouted someone when he was already outside.
"You’d better get your hair cut, it’ll weigh you down, especially at the top."
Vronsky was in fact beginning, prematurely, to get a little bald. He laughed gaily, showing his even teeth, and pulling his cap over the thin place, went out and got into his carriage.
"To the stables!" he said, and was just pulling out the letters to read them through, but he thought better of it, and put off reading them so as not to distract his attention before looking at the mare. "Later!" (Pt. 2, Ch. 20)
One of Vronsky's comrades, probably an officer, admonishes him to pay attention to his grooming. Wise advice! But does Vronsky really mind this advice? I don't think so. Because later on we learn that Vronsky never got that hair cut.
"Ah! I’m glad to hear it," said Vronsky. "Is madame at home or not?"
"Madame has been out for a walk but has returned now," answered the waiter.
Vronsky took off his soft, wide-brimmed hat and passed his handkerchief over his heated brow and hair, which had grown half over his ears, and was brushed back covering the bald patch on his head. And glancing casually at the gentleman, who still stood there gazing intently at him, he would have gone on. (Pt. 5, Ch. 7)
Notice how his hair has grown "half over his ears". This recalls an image of Karenin that we encounter early on.
"Well, God be with you," she said at the door of the study, where a shaded candle and a decanter of water were already put by his armchair. "And I’ll write to Moscow."
[Karenin] pressed her hand, and again kissed it.
"All the same he’s a good man; truthful, good-hearted, and remarkable in his own line," Anna said to herself going back to her room, as though she were defending him to someone who had attacked him and said that one could not love him. "But why is it his ears stick out so strangely? Or has he had his hair cut?" (Pt. 1, Ch. 33)
I think this repetition is not by accident. Early on, Anna wonders whether Karenin's ears stick out because of a hair cut. Later on, the narrator observes that Vronsky's hair has grown "half over his ears". I think this attention to ears and hair cuts is a little symbolic. Perhaps Karenin's ears stick out because he hears everything, and listens to everything he hears. His hair is short (which we may infer from the above passage) because he cares about appearances. On the other hand, Vronsky lets his hair grow out, and toward the end it is reaching his ears. Vronsky is aware of the chatter about his affair, but he chooses not to listen. He cares less about appearances.
Vronsky's hair serves to conceal, whereas Karenin's serves to uncover. Vronsky conceals his bald spot by brushing back his hair (pt. 5, ch. 7). Karenin's short hair uncovers the awkwardness of his ears, and the awkwardness of the whole affair. The idea of a bald spot as a blemish in his life of which Vronsky thinks too lightly, makes it the perfect symbol for his affair. Although it might cause other men insecurity, it doesn't make Vronsky insecure. He is far too confident for that. He is careless and cheerful about what other people notice and attend to. It's also worth pointing out, in the passage from pt. 2 ch. 20, the juxtaposition of the bald spot and the mare.