There are many auto-completion methods in Vim. For example, completion of words in the same file, the more intelligent omni-completion or completion of dictionary or spelling words. All of these completion methods have their own keyboard shortcuts. Remembering and invoking the right shortcut can be a hassle.
SuperTab is a Vim plugin that make all types of auto-completion easy. Install it for Pathogen by cloning its Github repository. To use just press Tab in Insert mode, in the places in your code where you need auto-completion. The familiar popup menu will appear with your choices and you will be able to proceed as usual.
Tried with: Vim 7.3 and Ubuntu 12.04 LTS