The shield is created by a fictional American metallurgist named Myron MacLain, who had been commissioned by the US government to create an indestructible armor material to aid the war effort. MacLain experiments with the vibration-absorbing metal vibranium.
Captain America vol. 5, #5 (May 2005).
During one of his experiments to fuse vibranium with an experimental iron alloy,MacLain falls asleep and awakens to find the experiment a success. This is due to an unknown catalyst entering the process during his slumber,and he is unable to duplicate the result. The vibranium-iron alloy mix is then poured into a mold for a tank's upper hatch to create the disc shape and painted to become Captain America's symbol. (Wikipedia.)
Originally, Dr. Myron MacLain was attempting to perfect an indestructible alloy as strong as the legendary 'Adamantine', which Hercules' Golden Mace was made from. Dr. MacLain was hoping the alloy would provide a distinct advantage in the armor of American war machines, during World War II. The Doctor worked tirelessly, using the exotic Vibranium metal and an unknown substance. While the Doctor was asleep, as a result of his exhaustion, an unknown factor caused the metals he was working with to bond. (Marvel Universe Wiki)
Therefore,it must be assumed,even in it's original,Pre-Fear Itself form,it was not JUST Vibrainium,but 'Vibrainium+Who-Knows-What'. Certainly Maclain never worked it out. Therefore speculating on what 'should' happen,should it be attacked with adamantium is redundant.It does what does,whether that be a property of the Vibranium,or some as yet unknown added substance.