When I clamp a long board in my face vice, the unsupported end of the board tends to sag towards the floor. To solve this, I added a couple of support pins to my workbench. One centred on the front edge of the bench, and the other near the end of the bench, as shown in the photo above.
Each pin is just a 5⁄16" whit x 150mm bolt with a smooth shank that’s screwed into a threaded wing knob. After cutting off the head of the bolt, it slides into a hole in the edge of the bench drilled at the same height as the face vice guide rods. To use the pins, I grab the knob and pull the bolt out just far enough to support my workpiece. When done, the pin slides back in.