The Vault stage is meant to ease players into the game which should include it's harsher sides. I agree that it is pointless if the vault learning curve serves no other purpose than to shelter the character from the outside world, but i think the idea is more to prepare them for it. So, since your trainers are all ex-wastelanders who know of its dangers and who have come here to get away from it, I would think that they would be training you for survival, giving you only what you need in order to surivive the waste.
as mentioned earning a level in a profesion is very useful in this way, teaching the character how to fish as opposed to giving them a fish they will loose as soon as they die for the first time in the waste. In this sense one really shouldnt gain many more possesions in the vault than one can in NCR already, after all, how much can the vault really afford to give away to begin with? but if the player has been taught to build and help themselves then loosing what they did emerge with wont be a huge issue as they already know they can reaccumulate everything on their own.
also they are not being kept in the vault, the original idea was that it would be optional from the get go and they can leave at anytime.
I and everyone else playing this game right now knows that the wasteland is a harsh place (much harsher than in the original game) and I for one would probably not be playing anymore if it wasnt for the help of the Wiki and my love of fallout which made me persistent to begin with. however while the Wiki is extremely useful i think that there can be a more organic way of being introduced to the new concepts of the game, through the game, instead of having to refer to a website continually.