The vibranium alloy that Cap's shield is made of is stronger than adamantium. Vibranium on its own is not, although it has other properties that still make it valuable (both the Wakandan and the Antarctic versions). No one quite knows how Cap's shield was made because its creation was an accident. Adamantium, I think, has actually been described as an inferior attempt to duplicate the process that made Cap's shield.
And yes, it can be dented in spite of its toughness and ability to absorb impact. You just have to be really, really, really, really, really, ridiculously strong to do it. Thor, when he had the Odinpower, hit Cap's shield so hard it got dented. He also popped the dent back out, restoring it to normal.