A small vice (or a very small pair of vice grips) so you can hold the screw on one side while unscrewing the screw on the other side.
I'm not sure a bench vise is enough as a tool to grip the screw's head because on some Streamlined cups, these screw's heads are convex-shaped and there is simply not enough "thickness" for a strong grip to occur. Better hold the cup into the vise and unscrew with 2 Philips screwdrivers. A dab of WD-40 to unlock might be useful. Inspect the tendon for cracks around the screws + axis's hole (inside the cups) every year; I usually change the tendon every 2-3 years depending on usage.
Agree with Ola : Never broke the Euro pin itself, the sping nor the base, but broke the M8 bolt flush that was tightened inside the pin. Main problem was corrosion of the stainless steel bolt, right at the junction between the stainless steel pin and the aluminum cup. Different metal composition can give rise to galvanic corrosion; pitting corrosion of passivated metal such as stainless steel can also occur (both are enhanced by chloride ions of seawater). Even if rust is not visible, care must be taken not to trust stainless steel too thoroughly. Change the M8 bolt every 3-5 years depending on usage since the bolt is under extreme mechanical stress. Same mechanical stress put on the bolt that tighten a US cup with a tendon joint, BTW...