"WHY VV3`S AVI FILE PLAY IN "VIDEO PREVIW WINDOW" IS MORE DARK THAN WINDOWS MEDIA PLAYER ?"

Short answer: Vegas is not Media Player. If your clips are destined for Media Player, you'll want to use the "preview in player" feature to optimize the look and sound for the destination player (WMP, or whatever-Real Player, QT player)

Long answer: Microsoft's DV decoder, which is what gets used in WMP, expands the normal encoded RGB range from 16-235 to 0 - 255 whenever it decodes it for display. While this method is okay (maybe even desirable) for displaying on a computer CRT, it is the wrong behavior for correct video processing. The CCIR 601 standard deals with "blacker than black" and "whiter than white" colors by narrowing the normal RGB range to 16-235, (220 quantization levels). If Vegas expanded the color range in the same way that Microsoft's decoder does it, we'd end up losing vital low and high range information. (e.g. test patterns and overexposed footage). As always, if the final output is intended to be broadcast or played on TV screens, you should always use a good external monitor setup when making color related editing decisions. Gamma response and range even on carefully calibrated computer CRT's are quite different from video monitors and TVs. If your final output format is intended solely for display on computers, you should use the "levels" filter along with the "histogram" view to "normalize" the pixel value range.