Yes, Vesta is technically an asteroid but...

One of my small goals is a more widespread awareness of how varied asteroids can be. In general we seem to understand quite well how terrestrial planets work as they tend to be somewhat similar in size to our own, as well as gas giants and stars - red/yellow/blue/white, dwarf/giant, that sort of thing. Visualizing the most massive stars is a bit beyond us, but besides that we tend to do a fairly good job. Asteroids though...maybe not so much. The one realization that doesn't seem to have taken place yet is that they can vary from tiny clumps of rock to small worlds, and the difference between the two can be phenomenal.

That's why the wording in this article today on the recently announced OSIRIS-Rex mission to asteroid 1999 RQ36 bugged me a bit. The mission will be a sample return, and the article compares it to Dawn by saying:

The OSIRIS-Rex mission is not NASA's first mission to an asteroid, but it will be the first U.S. probe to retrieve samples and return them to Earth...NASA's Dawn probe, meanwhile, is nearing the asteroid Vesta — the second-largest space rock in the asteroid belt. Dawn will orbit Vesta for many months, then head off to visit Ceres, the largest asteroid in the solar system. But the NEAR and Dawn missions are only visiting asteroids. OSIRIS-Rex will bring pieces back home.

Hm. See, the article mentions two things: Vesta is the second-largest space rock in the asteroid belt, and that Dawn is only orbiting it while OSIRIS-Rex is doing something more impressive. It seems to give the impression that while Dawn is visiting a very large rock, OSIRIS-Rex is visiting some other rock and will be doing something even more impressive. In reality it is doing something that can only be accomplished thanks to its target having next to no surface gravity of its own, because 1999 RQ36 really is just a large rock while Vesta is more aptly described as a protoplanet.

A simple graphical representation is probably the best way to explain this. 1999 RQ36 has a diameter of about 560 metres, Vesta 530 kilometres - 950 times greater. Ready? Here they are together. Vesta is black, 1999 RQ36 is red.

Can't see it? Let's zoom in a bit.

Can you see it there in red yet? Let's zoom in a bit more just to make sure.

There it is, a single pixel in diameter compared with the majesty of Vesta's 950.

In terms of walking time, to walk a distance equal to the circumference of 1999 RQ36 (about 1760 metres) it would take 21 minutes. To walk a distance equal to the circumference of Vesta (1650 km or so) - about six weeks if you walked eight hours a day. So yes, Dawn is 'just' orbiting Vesta (and then leaving and jaunting off to Ceres), but that's because Vesta isn't just any random chunk of rock.