What do you exactly means by using VIs as spectral enhancement... Any formulae that can extract information which is not easily receivable by naked eye can even act as an enhancement technique.  

Dear Saad, 

Vegetation indices (VI) are developed to estimate/predict amount of vegetation (cover, biomass, LAI, vigor/health, etc..) It uses 2 or more spectral bands (wavelengths) of vegetation in this case. Spectral enhancement such as PCA (which compresses redundant data values into fewer bands which are often more interpretable than the source data).  

In short VI are developed and calibrated in conjunction with field data to know accuracy of the model. But once developed minimum field data is needed for a particular site/vegetation. So depending on your source data (remote sensing) there could be a need for spectral enhancement or not.

Best, Mounir

I'll second what Atiqa Khan says. Vegetation indices are band ratios which by their very nature enhance the information content of imagery.  Just looking at an NDVI image you already know which areas are likely vegetated. That's what is meant by spectral enhancement. Because of this  enhancement aspect, you can classify a stand alone NDVI image by determining thresholds or you can add it as an extra input band when classifying multispectral. 

Dear Saad,

What kind of vegetaion index that you used ? With remote sensing data, maybe you can use familiar methode such as NDVI (Normalized Difference Vegetation Index) or EVI (Enhanched Vegetaiton Index). NDVI can help you to know the cholorophyl content of vegetation that is corelated to other aspect. EVI can help you to know the canopy density of vegetation. 

Thanks evryone, your answers were really valuable, i was litle bite confused about Vegetations index but now i think i've got  a clear idea about it.

by the way, im using NDVI.