First things first. VIM is more productive for programming than Eclipse. Your personal productity in VIM may be abysmal, but the potential cap of VIM is much higher. This is a fact.
VIM is a martial art. It feels unnatural when you first use it. And you can't even make it work. It takes years of practice to gradually become productive. You focus on mastering a little detail at first. Slowly all these bits you master add up until text is flowing effortlessly out of your finger tips onto the screen. Complicated edits that would make your co-worker sigh will jump from your hands before he can finish his exhale. There are few people who can use VIM. Fewer who can use it productivley. And you may never meet a master in your life time. But they are rumored to exist.
VIM is designed to keeep your hands on home-row. Moving your hand from the keyboard to the mouse is demoralizing. It's a gross motor movement. Moving your arm has a phsycological effect that hurts your motivation. Using VIM, someone could bolt your wrists onto the keyboard and you could still easily open up files, split windows, open tabs, build the project, search/replace, change fonts, change colors, etc. And all at lightning speed.
VIM is modal. That means you don't have to do complex key combinations where you hold down control+shift+Key. This hurts your hands in the long run. Instead you execute commands. There is no need for key combos to due to the modal nature.
We store data in our memory like computers do. Our memory can only hold a few values at a time. See how many distinct integers you can hold in your head before they start to slip away. We overcome this human limitation by writing stuff down. If data falls out of our memory we can easily look at what we wrote down to get it back. If your time is spent doing gross motor, physical things you are losing time that could have been spent on processing data in your brain. You want your mind to flow onto the screen without any effort at all. It may not sound like much but VIM's ability to effortlessly transfer what is in your mind to the screen is a BIG productivity boost. It's hard to put in words what I'm trying to say.
VIM supports code completion. Both textual and look-up based. It can pull text from mulitple files. Anything you desire can be had in VIM. Either make it yourself or use something someone else cooked up.
VIM supports goign to definitions with ctags. You can also find all references of an item. Again, anything you desire can be had in VIM.
The scripting of VIM is huge. You can download or create thousands of color schemes and change colors in an instant. Try to change fonts or colors in Microsoft Visual Studio and it will hang for 20 seconds while it loads data. It won't let you store color schemes and you must spend 30 minutes tweaking your colors and fonts every time you want a change of scenery. In VIM you can set line spacing to zero to fit more lines of code on the screen. I get over 80 lines. Visual studio uses 2 pixels of spacing for every line and you can't adjust it!!! Less lines = more scrolling = less productivity = forced to use small fonts for more lines = eye strain.
Split windows are opened in an instnat in VIM. It's usefull when you need to look at data in one section of the code that's far from the place you are typing (or in a different file). You don't have to spend time resizing windows, or worry about GUI windows overlapping each other and falling behind each other. Un-related code windows can be opened in tabs as to not take up screen space, but allow quick switching.
VIM as an IDE: http://www.youtube.com/watch?v=MQy2rVOf-z0&feature=fvwrel
VIM the revenge: http://www.youtube.com/watch?v=lQNFfhC4QI8
