Save in insert mode (VIM)

Try using Ctrl-[ instead of Esc.

What is the result of:

python -c "print ord(raw_input('char '))"

when you press Esc and Enter? It should be "27". What is the result of pressing Ctrl-V then Esc?

Have you checked all the settings in PuTTY to see if they're reasonable?

Are you using Bash on the remote system? Look at the output of

bind -p | grep -i '\\e' | less

and see if you see anything unusual. Do you have a file called ~/.inputrc? Look at its contents to see if anything is unusual.

Look at your ~/.vimrc and see if everything's OK, too.

Do you use vi compatibility mode? Try the command :set nocompatible.

In vi the insert mode notification is not shown on the status line (in contrast to vim) – so if you press i you actually are in insert mode but perhaps you just don't know it.

To save user-defined settings just put them in your ~/.vimrc (create if not existing), e.g. like:

set nocompatible