Is it possible to directly test whether of not the vacuum gravitates? According to GR, all sources of stress-energy (e.g. everything on the $T_{\mu\nu}$ side of the EFE) should gravitate (e.g. affect the curvature/$G_{\mu\nu}$ side of the EFE). We observe the expansion of the universe, which (for now) fits the expected expansion of a small cosmological constant. However, that does not mean that the expansion is due to a cosmological constant (the expansion may be a slowly moving scalar field, for ex., or something else). Also GR (while well tested) may be a limit of a more fundamental theory. Particle physics expects the cosmological constant to be much greater than what is observed in cosmology.
How would we go about testing whether or not the vacuum itself gravitates? The only way I can think of is something involving combining an Eot-Wash experiment with a casimir force measurement. 
p.s. The casimir force itself (as I understand) might not even be probing the vacuum.....there was a paper i saw a year ago explaining the casimir effect as the result of a van der waals force between the plates or something like that (i'd have to dig to find it if anyone is curious).
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How would we go about testing whether or not the vacuum itself gravitates?

Do we need to? General relativity is one of the best-tested theories we've got, see Clifford M Will's paper at http://arxiv.org/abs/1403.7377. And in the Foundation of The General Theory of Relativity Einstein did say this: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". The space around a star is vacuum, and it itself gravitates.  
One could argue that flat galactic rotation curves and gravitational lensing in the bullet cluster are direct tests of this. It's not as if anybody has actually detected any WIMPs. Let's not forget that we don't know of any breaches of conservation of energy, and that the raisin-cake analogy tells us that space expands between the galaxies but not within. That ought to suggest that vacuum energy density is now greater where the galaxies are. And we ought to expect it to act gravitatively in the same way as any other kind of energy. Space is dark, it has its vacuum energy, and this has a mass-equivalence. 
As for the cosmological constant, there's plenty of references where it's described as a negative pressure. A negative pressure is a tension. And where have we seen that before? In the balloon analogy: 

Image courtesy of the one-minute astronomer.
If you have a balloon in a vacuum, the pressure of the air inside is balanced by the tension in the skin, and there's two ways to make it expand. One way is to blow more air into the balloon. However energy = pressure x volume so this in breach of conservation of energy. But there is another way that isn't: make the skin weaker.  
