Vegging plants don't need any dark period. The number of hours of light they need each day are 24 minus however many hours of dark they're getting. Most flowering plants need at least 12 hours of darkness to grow buds.
No they don't. Keep it on a 24 hour light schedule untill your plant's are half the size you want them to end up. Then switch to 12 hours of light and 12 hours of dark and your plant will double size during this flowering period.
If you read up on the genetic makeup of the marijuana plant, they do need dark because a lot of metabolic changes happen to the plant during very low light and dark periods.. I personally have always suggested between 4-6hrs of dark in vegging... But options are like assholes... Everyone has one...
As Green said, they don't need a dark period. Some growers veg with a dark period and some don't. The benefits of 24/0 include more light energy for faster above-ground growth, shorter plants, not needing a timer and not worrying about light leaks. Vegging under 24/0 also makes for a bigger difference when switching to 12/12, encouraging the plants to begin flowering. Having a dark period will promote more root growth and use less electricity.