A vacume was used on my newborn baby during natural delivery. This has left a small pocket of fluid between the scull and skin how long to disappear?

Time. Can be quick or may take several weeks.Generally no harm from it however.
Usually two to three. 2-3 weeks. This usually not serious, vbut the cosmetic nature causes concern! not uncommon after pregancy and delivery with vacuum apparatus. Children (ie. Infants) heal quickly.