With just a week left in the month, Windows 8’s usage uptake has slipped behind Vista’s at the same point in its release, data from a Web measurement company showed.

According to Net Applications, Windows 8’s online usage share through Dec. 22 was 1.6% of all Windows PCs, an uptick from 1.2% of November. Windows 8 publicly launched on Oct. 26.

At the same two-month mark in Vista’s release timetable, that OS accounted for 2.2% of all Windows systems, double the month prior.

Net Applications measures operating system usage by recording the specific operating system and version used by the machines of visitors to approximately 40,000 sites it monitors for clients.

The slowdown in uptake of Windows 8 and its poor performance compared to Vista is a troubling sign for the new operating system. Vista has been labeled a rare Microsoft failure, in part because it was adopted by far fewer customers than either its predecessor, Windows XP, or its successor, Windows 7.

Vista’s online usage share peaked in the fall of 2009 at 20.3% of all Windows systems.

While there are nine days of Windows 8 data for December still to be released by Net Applications—including Christmas, when a substantial number of Windows 8 PCs may have been given as gifts, and thus not included in the online estimates—the new OS would have to record an amazing usage jump during December’s final week to put it on par with Vista’s 2007 pace.