This looks good to me. Vagrant/Virtualbox on testing are using the mainline
kernel guest additions, probably with a fake Virtualbox version.
You can see on the guest that the additions come from mainline kernel:
dpkg --search
/lib/modules/5.10.0-3-amd64/kernel/drivers/virt/vboxguest/vboxguest.ko
/lib/modules/5.10.0-3-amd64/kernel/fs/vboxsf/vboxsf.ko
linux-image-5.10.0-3-amd64:
/lib/modules/5.10.0-3-amd64/kernel/drivers/virt/vboxguest/vboxguest.ko
linux-image-5.10.0-3-amd64:
/lib/modules/5.10.0-3-amd64/kernel/fs/vboxsf/vboxsf.ko