The first frame shows a default vector plot.
To color vectors based on magnitude, set vcMonoLineArrowColor To False. gsnSpreadColors = True tells NCL to span the whole color map. In NCL version 6.1.0, you no longer need to set this resource to True, as the colors will be spanned automatically. In NCL versions 6.0.0 and earlier, the default is to start with the first color and use them in sequence.
New resources were introduced in version 6.1.0 to turn off the automatic span, and to specify a color palette to use with your vectors (independent of the workstation color map): vcLevelPalette and vcSpanLevelPalette.
veceff_2.ncl: Sets the size of the arrow heads.
Use vcMinDistanceF to thin your vectors based the distance between vectors.
You can also thin your vectors simply by striding the data:
vector = gsn_vector(wks,u(::2,::2),v(::2,::2),res)