The London Lounge

I have recently purchased a two piece RTW suit form a well known Savile Row tailor. Would it be very bad form and a bit uncouth to ask them if they could make me a waistcoat for the suit using the same material?

I don't see the problem here. This can't be a question of propriety given that they are already selling RTW clothing. And this would give them the opportunity to make an extra buck (or a few more).