  Vela X is the prototypical example of a pulsar wind nebula whose morphology
and detailed structure have been affected by the interaction with the reverse
shock of its host supernova remnant. The resulting complex of filamentary
structure and mixed-in ejecta embedded in a nebula that is offset from the
pulsar provides the best example we have of this middle-age state that
characterizes a significant fraction of composite SNRs, and perhaps all of the
large-diameter PWNe seen as TeV sources. Here we report on an XMM-Newton Large
Project study of Vela X, supplemented by additional Chandra observations.
Through broad spectral modeling as well as detailed spectral investigations of
discrete emission regions, we confirm previous studies that report evidence for
ejecta material within Vela X, and show that equivalent width variations of O
VII and O VIII are consistent with temperature maps within the PWN that show
low-temperature regions where the projected SNR emission appears to dominate
emission from the ejecta. We identify spectral variations in the nonthermal
emission, with hard emission being concentrated near the pulsar. We carry out
investigations of the Vela X "cocoon" structure and, with hydrodynamical
simulations, show that its overall properties are consistent with structures
formed in the late-phase evolution of a composite SNR expanding into a
surrounding medium with a density gradient, with ejecta material being swept
beyond the pulsar and compressed into an elongated structure in the direction
opposite the high external density.
