2 issues

According to this and many other sources:

The inclusion of an alpha channel in TGA format allows for the creation of images with varying levels of transparency…
Does Aspose Imaging support transparency with alpha Argb when saving Tga?

And also:

Dim TiffSaveOpt As TiffOptions
TiffSaveOpt = New Aspose.Imaging.ImageOptions.TiffOptions(Aspose.Imaging.FileFormats.Tiff.Enums.TiffExpectedFormat.TiffDeflateRgba)

Dim MyOptions As New Aspose.Imaging.ImageOptions.VectorRasterizationOptions
MyOptions.BackgroundColor = Aspose.Imaging.Color.Transparent
TiffSaveOpt.VectorRasterizationOptions = MyOptions

Returns one warning:
Variable ‘ASPSaveSetTIF’ is used before it has been assigned a value. A null reference exception could result at runtime.

Also using this:
TiffSaveOpt.VectorRasterizationOptions.BackgroundColor = Aspose.Imaging.Color.Transparent
Will actually throw null exception!

@australian.dev.nerds, Thank you for your request. VectorRasterizationOptions are only taken when converting a vector image to a raster image.
TGA is a raster image, unfortunately these options cannot be applied to it.
You can determine whether an image is a vector image as follows:

if (image is VectorImage)