Vectorworks 2015 was released prior to SketchUp 2015, therefore, Vectorworks 2015 did not include the ability to import version 2015 SKP files. The change in the SKP format from 2014 to 2015 was significant and is unsuitable for a service pack update.
As of the writing of this article, saving back to SKP 2014 format from SketchUp 2015 is the only way to get SKP files from Sketchup 2015 into Vectorworks, effectively the same method a user would employ to send SketchUp 2015 files back to SketchUp 2014, since it is also incapable of importing the 2015 SKP format.
This can be done in SketchUp by going to File > Save As, then after selecting a destination folder and file name, selecting "SketchUp Version 2014" (or the latest version compatible with the version of Vectorworks you have) from the Save As Type drop-down menu.
Edited by JimW