I recall hearing something similar, that you can't/shouldn't laser cut foamed PVC due to the risk of the fumes potentially killing both laser and operator...
But yes, I'd stick with mechanical cutting (power tools or traditional blood sweat and cursing).
I've had some succes with a ruler and sharp boxknife, but it's not fun going...
So, I'd support Sinewgrab: Tablesaw, router, or, if the friction doesn't cause heat/fume problems, maybe a scroll- or bandsaw?