Simple-R-Omni-Completion is a project mainly written in Vim Script, it's free.
Auto completes R identifiers from the current file and the included libraries
This is a mirror of http://www.vim.org/scripts/script.php?script_id=2656
Scans the current buffer for R identifiers and library() calls, offers autocompletion, first on the identifiers in the current file, than on the ones in the library. Identifiers within the current file are ordered by distance to the current line, standoffs are resolved by first suggesting those before the current line.
See also vimscript #2628 for a different approach to this.