@twostrokenut is right that a plant won't flower until it reaches maturity. While many indicas and hybrids will show preflowers, and even start flowering as early as 2 1/2 weeks, plants that take longer to reach maturity will remain in a vegetative state until fully mature. Just ask
@Uncle Ben about some of the true sativas he's grown.
As for re-vegging after harvest, it does work so long as there are a few bud sites left in the plant and as the plant is still in good health, which is rare since most people employ flushing toward the end of the flowering cycle. Normal cloning in my opinion is a much more "safe" way of preserving a specific plant, but re-vegging post harvest is a tool I use from time to time