The glasses show up in the first line of Neil's story:
The first time I saw Brenda she asked me to hold her glasses. (1.1)
This first line is telling us that Neil holds vision in his hands—this is his story, and it will be from his point of view. More specifically, Brenda's vision, her point of view, is in his hands. Since he's telling this story in the past tense, that makes him an ex-boyfriend, which makes him biased when it comes to certain issues.
As we discuss in "Narrator Point of View" and under the theme "Women's Issues," Roth exploits Neil's bias to highlight Brenda's point of view. The fact that the glasses are hers seems like an alert to the reader to try to see from her point of view even when Neil can't.
Her glasses are also a symbol of Neil's insecurity, and the resulting need to control Brenda—to make sure she doesn't get away from him. Brenda is quite aware of this, as we learn in the final blow-up. She exploits it most when she designs the sexy and (at least for Neil) dangerous game of hide and seek to make him say he loves her. When Neil waits for her in the pool during the game he thinks:
I wish that I had carried her glasses away with me, so she would have to wait for me to lead her back home. (4.80)
He wants Brenda to depend on him, to need him so she'll keep him around. Yeah, that doesn't sound too healthy. Anyhow, all of this is super ironic because Brenda is doing the same thing to Neil—trying to make him need her. In fact, she seems to be grooming him to be, well, her groom. Readers recognize what's going on, but Neil is far from sure. That's why he ultimately asks her to buy a diaphragm instead of asking her to marry him. He tells us:
I wasn't prepared for any answer but "Hallelujah!" Any other kind of yes wouldn't have satisfied me, and kind of no […] would have been my end. So I imagine that's why I proposed the surrogate […]. (6.35)
The "surrogate" he's talking about is the diaphragm. Notice the little shift to present tense. Neil says "I imagine." This shows us that he's looking back on and analyzing his past behavior, trying to figure out why he acted the way he did. Now he can share the irony with the readers and milk the situation for all the comedy it's worth.