Not sure if you're asking about the double-headed stake or big round stake in the leg vise: or the helmet.

More commonly the stake- that he's using to shape the helmet against, with a hammer- would be mounted in a square hole in the anvil, or ( today) one in a swage block... more probably (in helmet-wearing times) it would be stuck in a big piece of wood, like a section of a tree trunk buried into the shop floor. The stake has a tapered square shank, so it tends to wedge itself tighter into the block as it's hammered on.

For holding the helmet, there could be tongs. These could be quite specialized for holding different shapes for different jobs, and could also come with a ring to go over the handles, to hold them together so the smith wouldn't have to constantly squeeze. But here there would not so much be a need to have a really firm grip with the tongs, because he's working with pretty thin metal. The pair of vise grips he's hung onto it help him grip it a little better, but it's not like the work is hot and he really really needs them. He's raising it , or moving it, cold- and he has to, because it cools pretty quickly after leaving the forge. The reason for the forge is to anneal the steel periodically so it doesn't work harden from his hammering and stays malleable, and doesn't crack. There were small screw-clamp-like things called handvises , and I can imagine an amorer of the period using one where he's got those vise grips.

I wish I could give you a date for the invention of leg vises for blacksmiths, but can't. Certainly by the Renaissance, if not before.