Vi Versus Vim
Vi is a standard text editor that comes installed on just about every unix or linux system. It is a great tool to learn if you plan on troubleshooting unknown unix systems. Not all computers will have your favorite text editor installed, but they will all have vi installed. Imagine being hired to fix a machine, and then not even being able to open an simple configuration file. This nightmare scenario might happen if you do not know vi.
It is essential for system administrators to know at least the basic vi commands. If a system is having problems, you must be able to explore, view and edit any file on the computer. Vi is the basic was to do this on any unix system. Sometimes you will be forced to use the most basic tools to get more advanced tools working.
Vim is just an addition to vi. It adds more features and more commands. It is built ontop of vi and you can use all vi commands in vim (though not the other way around). Many unix systems now install vim by default and use a symbolic link to run vim whenever the user types 'vi' into the shell. There is no need to run vi if the system has vim, as all the vi commands work in vim, plus some extra commands.
Take a look at this vim cheatsheet for some useful commands you might not have known about.