When I'm using the command "/vanish",
I got this error In-Game, "An internal error occurred while attempting to perform this command."

And in the log I can see this:

2012-02-23 12:48:10 [SEVERE] Could not pass event org.bukkit.event.player.PlayerJoinEvent to VanishNoPacket
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1026)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:57)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:453)
at net.minecraft.server.ServerConfigurationManager.c(ServerConfigurationManager.java:133)
at net.minecraft.server.NetLoginHandler.b(NetLoginHandler.java:121)
at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:40)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:61)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1024)
... 9 more
Caused by: java.lang.NoSuchMethodError: org.bukkit.entity.Player.canSee(Lorg/bukkit/entity/Player;)Z
at org.kitteh.vanish.VanishManager.toggleVanishQuiet(VanishManager.java:225)
at org.kitteh.vanish.listeners.ListenPlayerJoin.onPlayerJoinEarly(ListenPlayerJoin.java:31)
... 14 more
2012-02-23 12:48:19 [INFO] [MCBans] MilkMan159 has connected!
2012-02-23 12:48:19 [INFO] MilkMan159 [/94.225.38.129:53769] logged in with entity id 31642 at ([world] -96.30000001192093, 74.0, 120.69999998807907)
2012-02-23 12:48:19 [SEVERE] Could not pass event org.bukkit.event.player.PlayerJoinEvent to VanishNoPacket
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1026)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:57)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:453)
at net.minecraft.server.ServerConfigurationManager.c(ServerConfigurationManager.java:133)
at net.minecraft.server.NetLoginHandler.b(NetLoginHandler.java:121)
at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:40)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:61)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:537)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:435)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader$103.execute(JavaPluginLoader.java:1024)
... 9 more
Caused by: java.lang.NoSuchMethodError: org.bukkit.entity.Player.canSee(Lorg/bukkit/entity/Player;)Z
at org.kitteh.vanish.VanishManager.hideVanished(VanishManager.java:239)
at org.kitteh.vanish.VanishManager.resetSeeing(VanishManager.java:119)
at org.kitteh.vanish.listeners.ListenPlayerJoin.onPlayerJoinEarly(ListenPlayerJoin.java:28)
... 14 more

I just installed the plugin from your dev.bukkit page, so it should be the newest version.
I will appreciate any help!

Regards!

As the download page clearly states you need 1.1-R4

I thought I had. -.-
Sorry. But thank you for your REALLY fast answer! :)

How do I get earlier versions of VanishNoPacket?

Upgrade to 1.1-R4. I'm not supporting any of the pre-1.1R4 releases. :)