Is there an "include" procedure in vimrc?

Yes, it is possible, use the :source command:

source .vimrc_module

See:

:help :source

for more information.

Plugins serve this purpose.

• plain plugins are always loaded once,
• autoload plugins are loaded on demand (they are ideal to define libraries of viml functions),
• ftplugin are loaded once per buffer (when the ftplugin type matches the buffer type),
• local vimrcs are loaded on a project-basis (non standard),
• and so on.

In other words, we hardly need to use :source or :runtime explicitly from the .vimrc. Yes they exist. Do we really need them? Hardly! Vim already has several idioms that take care of that.