NVIDIA announced it's not planning to bring Vulkan API support to its older Fermi GPUs, which were launched back in 2010. NVIDIA Engineer Jeff Bolz stated it's primarily an install base issue, there's nothing preventing the GeForce 400/500 series from supporting Vulkan but NVIDIA doesn't want to dedicate manpower to it because Fermi is already quite old and rapidly disappearing in the gaming market as people upgrade to more modern video cards.
In fact, NVIDIA has long been stating that Vulkan GPU support is offered on their Fermi cards but it seems like the issue is not with how these graphics chips were designed but rather with their install base. We know that NVIDIA captured a large market share with their Maxwell graphics cards and they had a large chunk of users who were Fermi owners upgrade to either Kepler or Maxwell. It should be noted that Fermi GPUs are already as much as 6 years old. The NVIDIA GeForce 400 series launched back in 2010 and was followed by the GeForce 500 series just a year later.