Your question would require more details. I assume you are trying to peen the tang over a pommel or washer plate? Did you make, or buy the blade? Is the tang annealed? Is this simple carbon steel? If the tang is hard, you cannot peen it. Heat the last inch or so of the tang red orange hot and let cool a couple of times, and try again. If you make the slot for the tang in the plate or pommel a close fit, It doesn't require a lot of upset to hold. When I do a through tang, I make a close but not too tight of fitted slot, then file the top of the slot hole to have light to moderate bevels, then peen, locked in the vise much as you described. It helps to bevel the edges of the tang also, as this helps prevent the tang edge from cracking as you peen a head on it. You should not have to use more than moderate hammer blows, if the steel is in an annealed state. If the steel is hard, it just ain't gunna happen.