what - literally, “of what nature” is your life? that is, how evanescent it is.
It is even - Some oldest authorities read, “For ye are.” Bengel, with other old authorities, reads, “For it shall be,” the future referring to the “morrow” (Jam 4:13-15). The former expresses, “Ye yourselves are transitory”; so everything of yours, even your life, must partake of the same transitoriness. Received text has no old authority.
and then vanisheth away - “afterwards vanishing as it came”; literally, “afterwards (as it appeared), so vanishing” [Alford].