If you see a vase in a book, you can bet it's going to break. And sure enough that's the case here, too.
But the vase that Cecilia and Robbie break at the fountain isn't just any broken vase. It's also… a symbol of their love. They break the vase on the day they come together just like their love is broken apart almost before it gets started. Get it?
The vase is a little more complicated than that, though. In fact the vase is not just a symbol but… drum roll please… a self-referential symbol. (Noticing a theme in these symbols? If not, be sure to check out our discussion of books and The Trials of Arabella.)
The vase was originally given to Cecilia's Uncle Clem during World War I by the grateful inhabitants of a French town he had helped to evacuate. The vase is valuable, but we're told it was "respected" not for its beauty or expensiveness, but "for Uncle Clem, and the lives he had saved" (1.2.12). The vase is important, in other words, because of the story that goes along with it.
Clem's isn't the only story that goes with the vase, though. There's also this story, right here, the one you're reading, in which Cecilia takes the vase down to the fountain, and then meets Robbie, and they break it, and then Cecilia takes off her clothes to retrieve the broken pieces.
And we get to see that story over and over. First, we see it through Cecilia's angry eyes (1.2.53), and then we see it again from a distance through Briony's uncomprehending but fascinated stare. We see the story unfold again through Robbie's memory, agonized with desire and love (1.8.2).
When Briony sees the scene, she imagines that she will write it down and "recast" it, "through Cecilia's eyes and then Robbie's" (1.3.28). This is, of course, exactly what happens in the book— Briony is giving us the plan for the novel she'll write and we're reading.
But the recasting doesn't end there. We also see the scene, repeated, in the novella Briony writes, which we have described to us in the rejection letter from her editor. And we finally see how the vase gets broken once and for all in 1940. Betty is carrying it downstairs and says it came apart in her hands, though no one believes her (3.19). We know, however, that the vase had already been broken and mended years earlier by Cecilia, so to us it seems totally possible that it actually did just come apart in Betty's hands. It was bound to fall apart sometime, just like Robbie and Cecilia's love story does when Briony reveals that they both died in the war.
So the vase is a symbol for the story of Robbie and Cecilia, and its falling apart and being put back together only to eventually fall apart mirrors their relationship quite nicely. The vase is also a symbol for symbols more generally, though, and how objects represent the stories we associate with them.