The directory structure for vcredist files changed between Studio 2015 and 2017, and windeployqt does not find the 2017 redistributables due to this.
In 2015 vcredistxx.exe used to be in a folder such as:
...\Microsoft Visual Studio 14.0\VC\redist\1033
In 2017 it seems to be in a folder like:
...\Microsoft Visual Studio\2017\Professional\VC\Redist\MSVC\14.10.25008
with version numbers being a variable.