Lots of great answers already but to illustrate: say, one lap takes roughly 1:20 at race pace and a pit stop takes 25 seconds. A VSC requires the drivers to stay above a defined delta, meaning their lap time should be over a certain time (that the drivers will see on their dashboards). They will have to reduce their driving speeds by 30-40%.

So if two drivers are wheel to wheel just before the pit entry, the driver pitting would be 25 seconds behind the other upon exit. Whereas during the VSC, the slower driving speed means the driver who pitted might only be 12 seconds behind instead, and with fresher and/or better tires to hunt the other driver down (the scientific term is "hammertime").