I'm not sure if Vim makes me more productive compared to other editors/ide's like Eclipse for example.
But somehow I get an empowering feeling when using Vim and noticed resistance to trying others editors. I've become an Vim addict.
Example: As soon a I see some cool feature in an other editor I'm thinking "Vi can do that (i just have to find the keystroke or configure a plugin)"
How can I benchmark editor productivity objectively?
Or "Once you go vim, you never go back"
If you like writing code in vim then that alone is a pretty good reason to use vim.
What good would a tool be that made you 2% more productive (according to some study) but that you didn't like as much? I tell you, working with tools you like is pretty darn important!
Why not just try out several different editors and see if you can notice any productivity gains from them. If you don't then pick the one that appeals to you the most, if you do you will need to decide if the productivity gain outweighs the happiness you feel when using vim.
You might also want to consider different editors for different frameworks/languages. I use vim for most of my C and Perl programming, a different editor for non-trivial Java applications, and another editor for developing in Rails, I haven't found a one-editor-fits-all yet.
To do it objectively, you'd need something measurable.
If you've got enough free time on your hands to experiment, I suppose you could video record yourself using each editor for a few hours then add up the length of time you spent fighting each one...
I also I'm addicted to the Vi input model, I'm sure that it makes me more productive.
I feel uncomfortable when I use some other editors. When I use Visual Studio I really need ViEmu, in Eclipse I use viPlugin, and so on.
Some time ago I was an Emacs user, now I can't use it without Viper.
However the productivity with Vi really comes when you are able to use commands without even thinking about them.
So, whatever editor you use, to get a real productivity gain the editor has to become an extension of your hands.
I'd say those symptoms are subjective clues that you're probably more productive in Vim - frustration with other tools is likely to be a fairly good indicator.
I would say with a huge degree of certainty that if you're into Vim deep enough to get frustrated with other editors, any productivity gain from switching is likely to be very small.