In the current situation Bumblebee users can not make use of vulkan because the profile does not install the required packages to be able to utilize either the Intel Vulkan driver or the Nvidia vulkan driver.
Bumblebee users can use vulkan with the pvkrun command without any additional packages to install.
Workaround / alternatives for the current situation
Users can install primus_vk and vulkan-intel manually.
Means of achieving / viability
I have successfully achieved this behavior by modifying the profile in my own custom ISO by adding primus_vk and vulkan-intel to the dependencies in the video-hybrid-intel-nvidia-390xx-bumblebee mwhd profile. This tweak seems highly viable to implement across manjaro as a whole.
** Risks / hurdles **
Updating the MHWD script means that the MHDW package configurations on the iso builder profiles need to be updated to include the new packages on the disk. There has been 1 report of nvidia-xrun no longer working with these 2 packages installed.