I am compiling Godot's master branch from source in MSVC 2022 for debug x64. I have a old laptop with GeForce 630M graphics card which does not currently have vulkan support. So I need to disable it so that the build uses something else like opengl3 which seems to be next in the search order.
If I do the build on the x64 Native Tools Command Prompt for MSVC 2022 with:
scons p=windows vsproj=yes vulkan=False
Then the generated exe works right away. But the vulkan=False flag is not written out to the vcxproj's Build Command Line. Because if I build from MSVC then the vulcan setting is enabled(the VULKAN_ENABLED macro is defined).

What would be a good way to go about fixing this?

Also the official release build(Godot_v3.4.4-stable_win64.exe) starts up fine, so it seems its not using vulkan.

The proprietary nividia drivers support Vulkan on the Geforce 6xx series. At least they did so under Linux and PC 2 years ago. You can verify (or falsify :-)) with running vulkaninfo.
Godot 4 uses Vulkan as primary graphics api, so vulkan=false will probably have no effect I guess.

Ah ok, its probably a driver issue then. I just updated to the latest drivers I could find for 630M: 391.35-desktop-win10-64bit-international-whql, and they do not seem to support Vulkan. I'll keep searching for more recent drivers though it seems, from what I read up, the Fermi architecture may not have gotten support.

===========
VULKAN INFO
===========

Vulkan API Version: 1.0.65

WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_api_dump.json invalid layer manifest file version 1.2.0.  May cause errors.
WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_gfxreconstruct.json invalid layer manifest file version 1.2.0.  May cause errors.
WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_khronos_synchronization2.json invalid layer manifest file version 1.2.0.  May cause errors.
WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_khronos_validation.json invalid layer manifest file version 1.2.0.  May cause errors.
WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_screenshot.json invalid layer manifest file version 1.2.0.  May cause errors.
WARNING: [loader] Code 0 : loader_add_layer_properties: D:\VulkanSDK\1.3.216.0\Bin\VkLayer_khronos_profiles.json invalid layer manifest file version 1.2.1.  May cause errors.

Instance Extensions:
====================
Instance Extensions     count = 11
        VK_EXT_debug_report                 : extension revision  9
        VK_EXT_display_surface_counter      : extension revision  1
        VK_KHR_get_physical_device_properties2: extension revision  1
        VK_KHR_get_surface_capabilities2    : extension revision  1
        VK_KHR_surface                      : extension revision 25
        VK_KHR_win32_surface                : extension revision  6
        VK_KHX_device_group_creation        : extension revision  1
        VK_KHR_external_fence_capabilities  : extension revision  1
        VK_KHR_external_memory_capabilities : extension revision  1
        VK_KHR_external_semaphore_capabilities: extension revision  1
        VK_NV_external_memory_capabilities  : extension revision  1
C:\VulkanSDKBuild\workspace\LoaderAndValidationLayers\demos\vulkaninfo.c:1670: failed with VK_ERROR_INITIALIZATION_FAILED

If it doesn't work out you'll need a newer PC. That graphics card wasn't the fastest back in the day, and is probably too slow for Godot 4 anyway.

Yeah I am just trying to drag this thing out to the limits. 10 yrs and counting now... 😃
So it seems Godot 4.1 will have GLES 3 support, so I am counting on that. Till then its Godot 3.x the LTS release I guess.