Is there a setting similar to the one found here for VMs ?
I cannot seem to set up a IPSEc VPN tunnel. I tried on Debian and Ubuntu guests, ending up with an error Cannot find device "ppp0"
If you're talking about QEMU/KVM virtual machines, all settings and completely up to the guest operating system and cannot be influenced (with respect to ppp) from the PVE side. I suggest loading the ppp kernel modules, but honestly, I haven't used ppp for a VERY long time. Have problem do you want to solve with it?