The default Rock style is

• no tabs
• since they are code out there using tabs, always keep the default of 8 character long
• indent of four spaces
• avoid long lines (a limit of ~80 characters is sound)

The most important point being:

when you edit a software package, follow the style of this software package

Vim

Corresponding vim configuration:

set shiftwidth=4
set softtabstop=4
set tabstop=8
set expandtab

Additionally, it is usually a good thing to enable syntax-specific indentation:

filetype on
filetype plugin indent on