Note: Be careful not to confuse a vertical line with the horizontal stabilizing grid wires described in Article 11497 "Apple Color Monitors: Cause of Thin Horizontal Line(s)" . The horizontal wires are completely normal.
The vertical line anomaly is inherent to this type of display when exposed to lower temperatures and is not indicative of the need for a service call.
If your display is exhibiting visible thin vertical lines, you can most often break the adhesion between the grid wires by using the heel of your hand to moderately impact the side of your display under the Apple logo on the right, then left side. Using moderate force does not damage your display.
To avoid scratching or marring the external plastics, remove any jewelry or watches before you tap the display.
If the lines persist after a few attempts to break the adhesion of the wires, you should take the display to an Apple Authorized Service Provider for additional troubleshooting.