I can talk more about our design intent for availability of attachments on Vault Weapons.
Our design intent for the Vault Weapons is to ensure they remain authentic to their Battlefield era. This means a Battlefield 3 weapon will not receive "new" attachments from 2042. It instead only has access to its era specific attachments.
Another aspect of this is that having 2042 attachments available on "older" weapons would look out of place visually. And in many cases, the 2024 attachments would also have the exact same gameplay benefits as their era counterparts, so it would make choice more complicated without benefit.