Different sources say different things. Marvel says Cap's shield is all Vibranium. Marvel Directory says it's an alloy of vibranium + adamantium. However......
Vibranium
Definition:
A precious extraterrestrial metal found only in meteor deposits in the African nation of Wakanda and in the now-frozen jungle of the Savage Land.
Emphasis on the word metal. So...if Magneto was able without such a horrible effort to simply pull the adamantium out of Wolverine's body, therefore changing it on a molecular level, i think it's safe to assume he could do the same to the vibranium. So...question is....can Magneto turn Cap's shield into a little puddle of Vibranium or Vibranium/Adamantium?
Because i personally think he can. And if he can, Cap's left without his precious toy. And worst of all, the process of creating the shield cannot be duplicated....
I'm not aware of that ultimates thing. However, yes, it is metal and therefore i don't think Caps could do anything to stop him. Rather a sad tragic moment IMO. Oh wait, the Ultimates reference you gave...does it have to do with Caps running around with that energy shield later on? (the one that looks like a hologram) ?
no... magneto attacks the avengers base, thor throws his hammer and magneto stops it easily, im sure cap also throws his shield... and mags says something to the effect of "who throws a shield?"
In normal continuity when Magneto was heading the New Mutants, the Avengers ambushed him and when Captain American threw his shield, he couldn't control it (something about the Vibranium) and it was Magik who was present that had to rid of the shield in the battlefield (teleported it to Limbo). This was in the 80's however.
Demigawd mentioned in the vs. post once that Joseph (clone of Magneto) was able to control Captain America's shield and with all the upgrades I keep hearing of Magneto, he should also have no trouble now.
Note: a robot poser of Magneto was able to grapple and shoot back the shield of Captain America's in Acts of Vengeance, but it was using something else to emulate Magneto's power.
I know of both the robot and Joseph. The robot was weaker than Magneto when it came to power. Joseph was stronger than Magneto as in raw power, but had incredible trouble controlling it and using it properly. Magneto would wipe the floor with him. And yeah, since the 80's Magneto's received many upgrades. I mean if the dude can control the entire Earth's magnetism, or create Black Holes, etc. etc....i think he could control the shield...maybe even destroy it as i said in the earlier post.