Square-cut waistcoats only work when they are double-breasted, IMHO. Like this one from Vox:
Single-breasted waistcoats almost always look better with the "inverted V" cut.
All suits (and clothes as a general matter) are costume.
As I have a very similar custom made suit to the one pictured (double breasted, peaked lapel low-button stance vest with single button peaked lapel jacket) I can tell you the whole point is to stand out. Is it ostentatious? Absolutely. In the last 100 years nobody wore such an outfit other than to be sartorially over-the-top. That said, I have received nothing but accolades when wearing it (and a growing number of requests on how to obtain a similar suit), so don't knock it 'till you try it.
Vox's 3-piece is beautiful, elegant, highly traditional in many ways, in a very restrained colour, and yet has a touch of the dandy about it in its cut. In other words, it's damn near perfect. Why does it worry some people so much? What's the worst that's going to happen if you wear a suit like this? The tedious, hyper-conservative and narrow-minded wouldn't talk to you, perhaps? That would be a bonus in my book. And in any case, most people would not even notice the DB vest unless the jacket was unbuttoned deliberately to show it, as in the picture.