A progression from V-vi at the end of a phrase is known as an interrupted cadence. It is called interrupted because the vi substitutes for I in what would otherwise be a perfect cadence. The submediant and tonic chords have two notes in common, but because vi is a minor triad, and the perfect cadence is such a strong resolution, the effect is quite striking.
Interrupted cadences are a common feature of the playful classical style, which constantly plays with the listener's expectations. Here a vi delays the expected arrival of I of a perfect cadence after the progression ii to V7 (when this phrase repeats, after the end of the example, the music does finally arrive on I).
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