If I remember correctly, Ubuntu 8.10 does not have the full version of vim installed as standard.
To remedy this go to Terminal and type:
$ sudo apt-get install vim
Press Enter, give the password and press Enter.
Alternatively, install vim using Synaptic.
An excellent vim tutorial is here:
At the expense of upsetting dyed-in-the-wool vi fanatics (apologies in advance), I recommend you use vim exclusively and drop vi.
vim is so much simpler and easier to use than vi. However, if you like a challenge then use vi.