One of the most common things I use an IDE for is navigation (between files) and vim is not very good at it compared to IDEs, here's why:
* no tabs
* no project structure
* in my IDE (eclipse based or intellij based) I can ctrl-click a class/object/other to navigate into that object's definition
* in my IDE I can search for classnames, I can press ctrl-T to find who inherits from something, ctrl-alt-H to find who uses something etc...
when I find myself actually writing code, it's usually a line or two in each of several files (because modern programming paradigms tell us we should split code as much as possible), or modifying some copy pasted snippet I found on google - so navigation is extremely important even when editing.
beyond that, there are simply some things that help writing code that a text editor cannot do:
* auto-complete(with annotation to tell you what the function variables are)
* debug inside your IDE
* run code-lint, auto-build on typing to detect errors, etc...
* auto refactor (I.E. extract method, move variable, extract class etc...)
* format all your files according to your pre-specified project guideline.
* sharing you IDE configuration with other contributors
* showing when files are/aren't in version control and integrating version control so that you view history in your IDE.
There are probably projects out there where a full-featured text editor is a real game-changer and is worth the days/weeks it'll take to use it intuitively, but I never worked or know anyone who worked on such a project (even when I started projects from scratch);