When you install a system of Rocky Linux, only the current needed drivers are enabled in the initramfs. Ubuntu very likely does not do this and has everything available in the initramfs. The implementation in virtualbox is virtio_iscsi, which exposes disks as /dev/sdX devices rather than /dev/vdX devices.
virtio_blk not working for you is not a surprise in this case, because it appears Ubuntu loads everything.
With the implementation that virtualbox is used, it is not a surprise you are running into this.
[root@xmpp01 ~]# lsmod | grep virtio
virtio_gpu 98304 0
virtio_dma_buf 12288 1 virtio_gpu
drm_shmem_helper 28672 1 virtio_gpu
drm_kms_helper 274432 2 virtio_gpu
virtio_balloon 28672 0
drm 782336 4 drm_kms_helper,drm_shmem_helper,virtio_gpu
virtio_net 81920 0
virtio_blk 32768 9
virtio_console 45056 1
net_failover 24576 1 virtio_net
My suggestion is before you convert the disk, you need to add the drivers to the initramfs. See below for an example.
% cat /etc/dracut.conf.d/virtio.conf
add_drivers+=" virtio_blk "
% dracut -f