I'm trying to make a vellum cloth object behave like a very tensile steel - like you would in a spring for instance.

In the vellum cloth constraint SOP it seems that setting the bend stiffness works only up to about 1000. Beyond that, my cloth still can bend and increasing it further (even up to 1+e12) has no more effect.

Reducing the polygons of course does make it stiffer to bending but I'd assume I could set the bend stiff to very high and it should act like a steel plank.

Thanks!

Try setting the Constrain Iterations to 1000, on the Velum Solver. More constraints makes it behave more solid, however, Vellum is not a unified solver intended to replicate completely solid objects.

scaling up the simmed geo (try X100 to see the effect) then scaling it back down after you cache it helps as well!