Unicode escape is ctrl+[ , why does this work in terminal or vim, but not safari or other parts of OS?
For example if search a page in safari you can't escape out of the search with ctrl+[


It's because Safari is not operating in an ASCII/ANSI Terminal environment.  

The Escape character is 


octal: \033
hexadecimal: \x1B or ^[ 
decimal: 27


It's just a series of characters the terminal looks for that interprets it as actions or special characters.

As for working in vim (or vi), you aren't limited to sending ^[ (^ Control[), you can simply press the Escape key.  It will do the same thing.