I made a little guide for that job. It's basically a V-Block with a slit cut about an inch into the V for the blade. Getting it aligned and clamped down is a chore, but once done it will cut round stock down the middle pretty accurately.
Depending on the accuracy you need of course you might try drilling a hole in a block of something (even wood) driving in the round stock and put this block up against a guide fence and cut the whole thing.
I too use a upside down drill press vice to do this. Ideally pick something with a nice level top like a toolmakers little vice. Remember too most bar will pinch shut as the cut gets deeper due to stress in the bar. Hence don't use to wide a blade or it will jam!