When Bev comes home from shopping in Palm Beach one day, she has a pineapple vase that she's purchased as a souvenir for Grandma Glad. The pineapple vase just seems like a passive aggressive—and hideously ugly—gift that Bev is giving to her not-so-beloved mother-in-law.
But really, the pineapple vase is a cover for Bev—in the same way that Evie is. When she goes out with Peter, she pretends to be shopping or chaperoning Evie, when really she's carrying out a torrid affair. The gift shop owner who saw her buy the pineapple confirms this:
She remembered them because they were both "so attractive." The woman was dressed all in white. They bought a pineapple vase. They laughed together as the man paid for it. (29.14)
The pineapple vase may seem like an innocuous, stupid tourist gift, but it turns out to be bigger than that. It's more proof of Bev's many lies—and when Joe hurls it, causing the vase to shatter, we know for certain that their marriage has hit a major rough spot. Bev's web of lies is falling to pieces, just like the pineapple vase she purchased to help cover for herself.