VC ++ 2015 cannot be installed over a newer version installed, that's Microsoft answer when you try to install, looking at my system installs a newer version was installed by windows 10 update 2/5/18.
Of course, if you already have a newer version of the SAME runtime (2015), there's no need to reinstall it. If it's working but, the 2015 runtime is not the one that usually have issues, because it's used by P3D4 itself.
But, as explained in all the threads I've linked, our software require ALSO the 2013 AND the 2005 SP1 versions (in order to be compatible with 32 bit P3D and FSX too) so, one of them might be either missing or corrupted, which is why, in the linked thread, I posted links for ALL 6 versions (2005 SP1, 2013 and 2015, in both x86 and x64 versions, and yes, you must install BOTH, even on a 64 bit system) so, it's up to 6 runtimes, since I cannot possibly know WHICH one is missing corrupted on your system.
And yes, a corrupted version or a version that is missing some files, WILL still reported as "installed" and no, just reinstalling wont' "fix" problems. Which is why, I always said to UNINSTALL and *then* reinstall every version of every runtime that is required but MIGHT be gone corrupted, usually because of other software, not necessarily related to flight sim.
So if this is the problem were Done for windows 10 and FSDT are not compatable.
They obviously are, otherwise thousands of users would have reported it by now, not just a handful. Everybody at FSDT use Windows 10, and we all have the latest update.
I just got a brand new laptop this week, which of course came with Windows 10 pre-reinstalled so, I had a chance to try a fresh new system so, I only installed P3D4 AND GSX and, of course, it worked at the first try.
Updated Windows to version 1803 and, of course, no problems whatsoever.