Take a look at the Command-T plugin:
Here's the description from the developer:
The Command-T plug-in for VIM provides an extremely fast,
intuitive mechanism for opening files with a minimal number
of keystrokes. It's named "Command-T" because it is inspired
by the "Go to File" window bound to Command-T in TextMate.
Files are selected by typing characters that appear in their
paths, and are ordered by an algorithm which knows that
characters that appear in certain locations (for example,
immediately after a path separator) should be given more weight.
As you browse, files are presented in a flattened list, with full pathnames relative to the current directory:
To narrow the list of files, type characters from anywhere in the pathname. For example,
bul are enough to specify
Once you've typed enough of the pathname to find the file you want, just hit Enter to load it. (You don't have to get all the way to a single file; once you're close, you can use the arrow keys to select the one you want.)
Anyway, it's not exactly what you described, but it might be exactly what you want. :-)