Sorry I don't own a Vepr, but I do own a Saiga 223, which is manufactured on nearly identical tooling and processes, as Izhmash and Molot most likely manufacture barrels on the same equipment. The Saiga is simply a lighter barrel than the Vepr. They are both 1/9 twist.
I have never really taken the time and effort to do what is necessary to get the rifle to shoot tighter than 2MOA or so when scoped, and with that standard of performance it shoots about the same with all grain weights - 55gr, 62gr, 75gr. It may be possible that if I put some attention into handguard tension, removed the cleaning rod, and cleaned up the FCG it may tighten up with one particular load, but 2MOA is more than enough out of an AK to satisfy me.
I've had a good amount of AE 50gr tipped varmint rounds, the Saiga doesn't like them and doesn't stabilize very well, they're all over the paper.
One person on another forum I'm on had a few Saiga 223 rifles and had them slugged out of the box - the bore diameters varied by a couple thousandths. Most likely due to chrome plating inconsistency.
As far as brass vs steel, the rifle will most likely not care. The only thing I would say about that is that you'll most likely get better QC on any brass-cased .223/5.56.