Onward and Upward
Except for two key scenes, Vertigo's story is told in a chronologically linear way, despite all its twists and turns. The exceptions to this rule are (1) Scottie's dream sequence and (2) the flashback with Judy's voice-over letter revealing her part in the murder of the real Madeleine.
The dream sequence affords us a glimpse of Scottie's damaged inner life after the death of "Madeleine." In his nightmares, traumas seem to repeat themselves with a kind of timeless regularity; the sequence represents a break from the forward movement of the film's story.
The flashback is where we're given the key that Scottie's been seeking in trying to understand what happened to Madeleine. We're seeing what went down before the present moment in Judy's apartment. We're not just hearing about what happened in the past (which happens lots of times throughout the film, as information about Carlotta is given out in in dribs and drabs); we're actually witnessing this past thanks to the flashback technique.
Even though the story moves forward, it doesn't always seem that way because of the way Hitchcock tells it. As New York Times critic Terrence Rafferty put it, "This movie isn't constructed, as most thrillers are, to get us from point A to point B as swiftly and as efficiently as possible. "Vertigo" instead circles compulsively around a set of visual and verbal (and musical) motifs—spirals, towers, bouquets, the words "too late"—which keep bringing us back to the same places, turning us in relentlessly on ourselves." (Source)