I agree, but I wanted to clearly under estimate because three or four days is already enough cost wise and it's hard to argue. I think two weeks is on the optimistic side, but I think real proficiency would be a slow process and likely end up not paying off for about a quarter, though you'd be productive in other ways during that time.
If you're a proficient IDE user, learning and setting up Vim up to a comparable level to a top-notch IDE (Visual Studio, IntelliJ) would take more than three or four days. Three or four weeks (or even months sounds) more realistic, in order to get efficient:
* code navigation * code refactoring * debugging * tool integration * workspace/project management
Don't let anyone tell you otherwise, I'm a Vim user and the people that say it will take just days or a few weeks have just forgotten how long it took them to ramp up. Or they're fooling themselves that their Vim does everything a capable IDE does on strong hardware.