Disclaimer: These ideas are based solely on my opinion, please feel free to ignore, flout and disobey at every opportunity.
There has been an interesting pattern this month - I was expecting most new orders to be for casual jackets, items to dress up a casual look when needed. But to my delight, you have proven once again that the suit is not dead. Far from it.
There have been all manner of traditional cloths passing over the board this month, navy birdseyes, checks, even a couple of pinstripes. And more interestingly I have seen a huge increase in three-piece suits! Not exactly casual tailoring.
It seems that after months of trackies, the roaring 20's is going to be very dapper indeed.
So what do you need to think about with a waistcoat?
The most important thing is length.
A waistcoat is designed to cover the messy area where your shirt is tucked into trouser. So it must reach the seam of your waistband.
With many men wearing their trousers on their hips these days, that means that waistcoats must be longer (or trousers must be higher).
The second thing is neckline. Please can I make an impassioned plea against the horseshoe collar. It looks ridiculous. It makes one look pigeon-chested. And similarly, don't have them cut too low. A waistcoat should leave no more than 3 shirt buttons showing.
If you want to wear it without a jacket then opt for a more casual style, more like a gilet with a Nehru collar or you risk looking like an estate agent or bouncer.
Otherwise, the fastening should mirror the coat: single-breasted -> single-breasted, double-breasted -> double-breasted.
Collars can be a really lovely addition but definitely make it more formal. If the suit is linen/cotton/summery etc then I would forego the lapel.