ubuntu12.04 when installing VIM, there is currently no available package vim, but it is referenced by other packages

The following prompt appears during installation:

There are no packages available for Vim now, but it is referenced by other packages.
This may mean that the missing package may have been discarded,
Or can only be found in other publishing sources

E: Package vim is not available for installation candidate

Workaround:

First step: sudo apt-get update

Step two: sudo apt-get install vim

ubuntu12.04 when installing VIM, there is currently no available package vim, but it is referenced by other packages