Ctags Usage:

vim -t subroutine_name 
:tag subroutine_name
ctrl-] jump to definition of subroutine under cursor.
   All will jump to appropriate file.

ctrl-t or :pop to jump back
(or just use ctrl-i and ctrl-o to jump between recent input points)

Configure tag file for project:
:set tags=tags\ /path/to/tags/for/project.tags