Personally I didnt think Ultron was as invulnerable as he should have been, and if he did put some Vibranium in him, Id say it was a Full Body makeover, otherwise I dont see him being damaged (even by combined blasts from Thor, Vision and Iron Man) as he was at the end. Caps Shield is pretty much indestructible (say possibly for Thanos, as per Tonys vision), I didnt see Ultron with that level of durability, personally.