Howdy
Sometimes nickel plated brass tends to split more often than plain brass. This can happen if the plating job has been poorly done. I have never used any Starline plated brass, but I have used tons of their standard brass, and it is top quality, so I suspect their plated brass would also be top quality.
However I must ask, with your black western rig and stainless New Vaquero, do you have any idea how much a gun belt with 25 or so rounds of 45 Colt in the loops weighs? When I started in CAS I kept 25 rounds of 45 Colt rounds loaded with 250 grain bullets in my belt loops. Gave that up pretty early when I realized how much all that lead weighed. Of course, we carry two revolvers in CAS, but even so, putting that many rounds in your belt loops will easily add another pound to the rig.
Wearing two Colts at about 2 1/2 pounds each was plenty for me to carry around all day without the added weight of bullets. These days I only keep a few 44-40 rounds in my belt loops in case I need a reload.