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If your geotiffs are opened as separate files, they will not be used together in the VI calculator. You should save them together in one multiband file using either File ->Save File As ->ENVI standard (if they all have the same dimensions, pixel sizes, projections) or Basic Tools ->Layer Stacking (if they have different dimensions, pixel sizes, projections). Then, when you have one file with multiple bands, you can enter the center of the wavelength range in the file header attributes.
Depending on your number of bands and where they are located in the spectrum, you may only have a few VI's that can be calculated. The VI calculator was designed to be used with hyperspectral data and most of the indices are defined by very narrow wavelength ranges.
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