I actually find Vault to be very helpful. I use it mainly to read the employee surveys of smaller investment/research boutiques, to get a sense of compensation/morale.
The message boards aren't that informative. There are 10 questions asked for every 1 answer given, and many company boards suffer from grossly outdated information and a scarcity of posters. Although mostly useless, the boards can occasionally contain some tidbits of invaluable information --I saw a post that accurately described almost every question I was asked at a Lehman research associate interview.
In short, if you have a lot of industry contacts, and know how to play the game, then you probably won't gain much from a Vault subscription; for the rest of us (myself included), it can be a great resource.