Vinyl siding gap on corner
Some corners like yours are actually two parts--an inner part that's nailed to the wall framing, and a snap-on cover. You can actually see the inner component in your photo at the top. This arrangement allows for various styles to be used. Generally the inherent tension and friction of the two components prevents slipping like you've seen.

With a wooden block held snugly against the bottom of the slipped corner piece, use a rubber mallet or other tool to gently tap the slipped corner back into place. This would best be done on a warm day when the vinyl is less likely to shatter.

A small screw or rivet in an inconspicuous location, or a dab of exterior construction adhesive behind the cover, will help prevent future slipping.