I first posted a ticket here:
http://dev.bukkit.org/bukkit-plugins/authme-reloaded/tickets/481-vault-permissions-bukkit-null-pointer-exception/
but now I think it's a problem of PermissionsBukkit.
The problem is explained in the ticket on top.
Vault uses .getPlayerInfo(player) and gets null because a player is not mentioned in the config file (causing my error).
(see https://github.com/MilkBowl/Vault/blob/bd9d089a3e0068b43eee91a5ac3bd6e136df82aa/src/net/milkbowl/vault/permission/plugins/Permission_PermissionsBukkit.java#L161)
Shouldn't it return a PlayerInfo with group default as said here?:
"If a user is not specified here, or does not have a 'groups' node, they will be in the group 'default'." (see http://dev.bukkit.org/bukkit-plugins/permbukkit/pages/configuration/)
If its allright, I'm sorry. If you think Vault is wrong, I will post a third ticket there :D
Thanks for your help!