Try a Backpack
I purchased a LowePro backpack for my Mamiya C330 system 10-15 years ago. It works very well, not too big yet capable of camera, multiple lenses and the same size Vivitar flash that you mentioned. Also extra zippered compartments for filters, film etc. See photos. Unfortunately, I cannot find this model on LowePro's website and it is not marked, but I remember it being a "Trecker" of some sort. It measures approx 16 in tall x 13 in wide x 9 in deep. It is heavily padded including the carrying straps. I usually carry it around with one strap over my shoulder.
I purchased a LowePro backpack for my Mamiya C330 system 10-15 years ago. It works very well, not too big yet capable of camera, multiple lenses and the same size Vivitar flash that you mentioned. Also extra zippered compartments for filters, film etc. See photos. Unfortunately, I cannot find this model on LowePro's website and it is not marked, but I remember it being a "Trecker" of some sort. It measures approx 16 in tall x 13 in wide x 9 in deep. It is heavily padded including the carrying straps. I usually carry it around with one strap over my shoulder.