Any skilled tailor will be able to make a bias-cut waistcoat. The wool will be stiff enough not to cause draping trouble even on the bias, at least for such a small garment.

However, a skilled tailor could also make a straight-cut waistcoat with the same sett element center front as on your kilt... That uses a lot less fabric.

For myself I'd make it with a plain back, make the back from lining material. I'd probably go for what I call basque linen (cotton warp and linen weft, thin and pliable but springy), but I don't know if that's traditional and I have no English name for the cloth...