Packer config for the boxes which can be found at https://app.vagrantup.com/russmckendrick/boxes/centos8.
To build both Virtualbox and VMWare boxes run;
packer build CentOS8.pkr.hcl
For just VMWare;
packer build -only vmware-iso.vmware CentOS8.pkr.hcl
And for Virtualbox;
packer build -only virtualbox-iso.virtualbox CentOS8.pkr.hcl