(This is not a direct answer to your question, but I think it is even better :)
You should check out the Command-T plugin inspired by TextMate 'Go To File'. He selects the possible combinations very intelligently, just enter a few characters of each subdirectory, sufficient to distinguish it, and he "gets" it, the characters should not be at the beginning and do not have to be sequential. It also shows you a list of remaining options.
I understand this is a terrible explanation, so watch this video to see how it works.
The disadvantage is that Vim must be compiled with Ruby support.
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