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Does air affect a magnetic field's ability to attract ferromagnets? If a magnetic field and a ferromagnet were placed in a vacuum would there be a better performance?

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The magnetic permeability of air is negligibly different than vacuum, so probably not. – DumpsterDoofus Feb 14 '14 at 4:24
@DumpsterDoofus please explain this a bit more. – XCIX Feb 14 '14 at 7:50

Oxygen in air is a triplet diradical, ↑O-O↑, a a paramagnet. Its slightly greater magnetic permeability than vacuum would slightly concentrate the field. Compare magnetic susceptibilties,

Nitrogen (1 atm) $-0.5\times 10^{-8}$
Oxygen (1 atm) $209.0\times 10^{-8}$

enter image description here

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The configuration of a static magnetic field coming from a source object like a bar magnet is determined by the constitutive relations of the surrounding material, and in particular the magnetic permeability determines how the field lines act.

In the first case, you have a field emanating from a magnet, traveling through air ($\mu_r=1.000000349)$, and entering a ferrous material ($\mu_r\approx4000$).

In the second case, you have a field emanating from a magnet, traveling through vacuum ($\mu_r=1)$, and entering a ferrous material ($\mu_r\approx4000$).

The two situations are nearly identical, so there will be no change in performance.

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