Acronyms are one of the best parts of VA, and help me effectively mask the fact that I can't spell!
However, to use 'Open File in Solution', you have to type the name correctly to get it through the filter. It would be cool if the same acronym search used for variable names could be used to sort and filter files in the solution as well. Then you wouldn't necessarily need to know exactly what the files were called or how to spell them.
The dialog supports filtering by multiple substrings so that you don't need to know the exact filename. You can approximate acronyms by adding a space between each letter you type, though admittedly that does not filter on order of letters typed.
If filename is MySubString.cpp, you can type "st su" to help locate it.
Have you tried eliminating files that are not the ones you want? I often do this when I remember that it's in there somewhere, but am not sure of the details. For example, start with:
-.h -editor -\library
which will filter out all header files, all files with editor in the name, and all files with a directory starting "library" in their path.