You can use FileVault in combination with the screen saver lock feature. However, it is true that using the screen saver lock does not log you out and, therefore, does not "close" the vault.
Therefore, you should not rely on the screen saver to protect your computer over a long period of time — it is a convenient way to protect it when you are away from your keyboard for a few seconds but has not been designed to fulfill the same purpose than FileVault.
FileVault does not change the normal Mac OS X permissions system and the usual permissions-related safeguards continue to work normally.
Let me know if this helps !