A few things to consider here. The Wireshark installer attempts to install a copy of the VC++ redistributable, and MS is less than helpful in ensuring the return values make sense, often returning an error code that actually means the installation succeed but with a minor observation, e.g. a newer version exists. The Wireshark devs have been playing whack-a-mole trying to handle this.
As you're building an old version from the old-stable branch (3.4.12 is the latest for old-stable) you may be missing fixes that would correct this issue, so please try a build using 3.4.12. You might also consider moving to the current stable branch and give your users the benefit of all the fixes and improvements added to the current build 3.6.2.
The version of the vc-redist used by the installer is usually obtained from the Visual Studio installation use to build Wireshark, the CMake configuration step prints out the path in use, it's possible there might be issues with the vc-redist you're currently using depending on the specific updates you have applied to Visual Studio.
As you state you're distributing to a customer, if using npcap, you should ensure compliance with the license for npcap which has restrictions on distribution. Wireshark itself relies on the GNU General Public License, version 2 for distribution which you should also observe.
