Suppose you have a file with a list of cells, referenced by a VLOOKUP

choicea filea.xls
choiceb fileb.xls
choicec filec.xls
(there are actually 25+ files, but you get the idea)

Off of a menu, you choose choicea, choiceb, or choicec, and a bunch of cells
fill in, but the most important thing I want to do is update macros and
cells to read from the file selected above - so if I click on "choiceb",
read from file "fileb.xls". The problem is that I haven't figured out -
through either VBA or a formula - to get a formula to read from cells in a
different file.

Please keep in mind that this file could have up to 20,000 rows, with INDEX
and MATCH statements against any of 400,000 cells in those files, so what I
really want to do is force it to a variable, and then the formulas read from
a variable. I tried this and got the exact formula I wanted in the cell,
but it was interpreted at all.

I will leave this to the pros to see if they have an idea.

Thanks,
Brandon