I had this problem with the x86 redist and was eventually able to blame everything on the MS supplied vcredistx86 product.xml file. You should be able to apply the same fix to the vcredistx64 product.xml file. The problem arises due to file name and public key mismatches with the actual redist file that you download from MS.
You must change the name in both the PackageFiles PackageFile node and Commands Command node of product.xml to vcredistx64.exe, as well as renaming the redist package from MS to vcredistx64.exe. Then, change the publickey attribute in the PackageFile node to match the public key of the EXE you got from MS. You can get this by looking at the EXE's SHA1 certificate with File Explorer (Properties - Digital Signatures (SHA1) - Details - View Certificate - Details - Public Key).
The Visual C++ 8.0 redist packages (vcredist_x86.exe, vcredist_x64.exe and vcredist_ia64.exe) support the following command line installation options. The examples below use the file named vcredist_x86.exe, but you can substitute the 64-bit versions of the EXEs with equivalent command lines to achieve the same behavior for them as well. Vcredist_x64.exe problems include high CPU usage, application errors, and possible virus infection. Here are the top five most common Vcredist_x64.exe.