What does "envacuuming" mean?

They picked their way among the mummied figures. The black skin stretched upon the bones and their faces split and shrunken on their skulls. Like victims of some ghastly envacuuming.

IMO, McCarthy is employing the prefix en- to conjure up a vision of corpses which have been vacuum-ed from the inside, causing their skin to stretch tautly, and faces to shrink. Ugh!

In its entry for the prefix en-, NOAD says:

1 forming verbs (added to nouns) : expressing entry into the specified state or location (as in engulf)
2 forming verbs (added to nouns and adjectives) : expressing conversion into the specified state (as in encrust, ennoble).

Given that information, I'd assume that the word meant that the victims were somehow put into a vacuum. (Whether that alludes to a physical vacuum or a metaphorical one, I'd need to see more than the five words you've supplied in your post.)

Real word or not, others have used the term before. E.g., Hugh Kenner to Guy Davenport in a letter of November 1963:

"Columbus knew he had hit a barbarous bit of China coast; later voyagers knew they had hit an annoyance, an obstacle on way to China. It was by a major effort of the Renaissance imagination that the obstacle was transformed in the New World. Hence Renaissance megalomania part of the U.S. birthright, leading to Jay Gatsby, who is an envacuumed Medici."