Hafez,

The color function and the scale function both get 5 arguments: x, y, vx, vy, Norm[{vx, vy}] from these inputs you can calculate angle or any other value. For example, the vector angle would be ArcTan[vx,vy]

As to the line thickness you can specify the thickness (and many other attributes) with

VectorStyle -> Thickness[.01]

I do not see how you can specify the vectorStyle in one plot based on a function, however, You can use the RegionFunction to plot your vectors in different regions with different attributes.

Here is an example:

one = VectorPlot[{y, -x }, {x, -3, 3}, {y, -3, 3}, 
   VectorScale -> Medium, 
   RegionFunction -> 
    Function[{x, y, vx, vy, n}, x <= 1.5 || Abs[y] >= 1]];
two = VectorPlot[{y, -x }, {x, -3, 3}, {y, -3, 3}, 
   VectorScale -> Large, 
   RegionFunction -> 
    Function[{x, y, vx, vy, n}, Not[x <= 1.5 || Abs[y] >= 1]], 
   VectorStyle -> {Thickness[0.01], Red}];
Show[{one, two}]

to get a region with thick, red vectors and thin blue vectors everywhere else.

Hafez,

You should post an example so people can help you more easily. I grabbed one of the examples to demonstrate.

One way to highlight is with color functions: (note that VectorColorFunctionScaling must be false to get the color you specify)

VectorPlot[{y, -x }, {x, -3, 3}, {y, -3, 3}, VectorScale -> Large, 
 VectorColorFunction -> 
  Function[{x, y, vx, vy, n}, 
   If[x <= 1.5 || Abs[y] >= 1, Black, Red]], 
 VectorColorFunctionScaling -> False]

To get

to make them bold use VectorScale's function option: (note scale value is between 0 and 1, or it will autoscale)

VectorPlot[{y, -x }, {x, -3, 3}, {y, -3, 3}, 
 VectorScale -> {Large, Automatic, 
   If[#1 <= 1.5 || Abs[#2] >= 1, 0.3, 0.8] &}]

To get:

You can further adjust the options to get what you want.

Regards,

Neil