first off as long as you havnt done any topping, fimming, or trimming within the past week or 2 then theres no reason the transplant would hurt, simply transplant into the new home, fewer transplants is better on the plant so you may want to just jump to the 5gal pots right away, just remember after they are in their new homes all cozey and tucked in nicely give them a little watering of just plain pHed water.
as for length of veg thats dependent on a few things, mainly YOU, the longer you veg the larger it will be, there for with proper training you can increase the yield quite a bit, as i said proper training, if you top or fim or even prune (trimming leaves and branches to let light penetrate deeper) you MUST give them a week or 2 to recover before you do any other topping, fimming or pruning, now with tying branches down you can do that when ever, its actually very low stress as long as you dont force branches down causing them to break, slow bending, tie to a point and later you tighten up those ties to pull the branches down more after they have adjusted to their new positon. when it all comes down to it make sure you give them at least 2 weeks to recover from any topping, fimming, or pruning before you put them into flower, if they arnt recovered fully you will have problems in flower. but all in all its up to you on how long you veg for, i know people who swear on 2 month vegs and other who simply take them to 25 day reguardless if from seed or clone, so its all up to you in the end.