Q:

VIMRC not reflecting changes to tab spacing

I'm looking to modify my vimrc file so that when I press tab it indents 2 spaces instead of 4.
Currently I am using SPF13 
I have changed the following lines in my vimrc:
set shiftwidth=4
set tabstop=4

into:
set shiftwidth=2
set tabstop=2

then restarted my vimrc by using :so $MYVIMRC.   
When I reopen Vim and press tab it is still tabing 4 spaces instead of 2.

A:

To explain why it happened the way it did, softtabstop determines how many spaces is inserted via the Tab key. What is actually inserted is a mix of tab characters and space characters.
tabstop determines the number of spaces "occupied" by each tab character.
So when you press Tab, Vim tries to insert a number of space equal to softtabstop using tab characters, then pad the rest with space characters.
Examples:

softtabstop = 4; tabstop = 2; insert 2 tab characters.
softtabstop = 2; tabstop = 2; insert 1 tab character.
softtabstop = 6; tabstop = 4; insert 1 tab character and 2 space characters.

