try this
device.RenderState.AlphaBlendEnable = True
device.RenderState.SourceBlend = Blend.SourceAlpha
device.RenderState.DestinationBlend = Blend.InvSourceAlpha
device.TextureState(0).AlphaArgument1 = TextureArgument.TFactor
device.RenderState.TextureFactor = Color.FromArgb(alpha, 255, 255, 255).ToArgb
set the parameter alpha from 0 to 255 to set trasparency
hovever the best method remain pixel shader