The Disney Vault is used to generate excitement about a film in later years. The Lion King came out of the vault on 4 October 2011 – for its 3D release (which I missed, shamefully). You are right in saying that
TLK will be locked up on 30 April 2012, but that doesn't mean the film will become inaccessible. It will always be available through sites like Amazon or Ebay – although prices may be higher.
I doubt it will be 'vaulted' permanently though, Disney will want to build the excitement and release it in 8-10 years (possibly as
8K 3D, which would look absolutely spectacular in a large cinema).