call SetUnitVertexColor(255, 255, 255, your_alpha)
Your_alpha should be in range of 0 to 255, so you have to convert it, like with function like set your_alpha = PercentTo255(some_alpha_in_percent)
.
If you call SetUnitVertexColor and change the values of red, green or blue for the unit, you have to keep track of that yourself(There is unfortunately no GetUnitVertexColor)