How can I create a VM in Vagrant with VirtualBox with two CPUs?

Add vb.customize ["modifyvm", :id, "--ioapic", "on"] to the config.vm.provider block inside your Vagrantfile.

Looking at the VirtualBox documentation it mentions:

“Note Enabling the I/O APIC is required for 64-bit guest operating
systems, especially Windows Vista; it is also required if you want to
use more than one virtual CPU in a virtual machine.”