How to get a proper prefix length from DHCPv6 server?

Solution 1:

As per the DHCPd documentation:

In a properly functioning system the client should use a prefix length of 128 and get any on-link information from the RAs from a router.

Since you are using two virtual machines and I assume no IPv6-capable virtual router connects to that virtual network, nobody is sending out those RAs so your client cannot get the correct subnet mask.

Check the link for more information.