I just looked into this yesterday and found a reddit thread saying that Proton submitted the change a while ago and is waiting for Mozilla to approve. Let me see if I can find it again.

Edit: https://www.reddit.com/r/ProtonPass/comments/179c6uz/comment/k59a04d/?share_id=SB6uhJMiGs2IbyZzNji33&utm_content=2&utm_medium=android_app&utm_name=androidcss&utm_source=share&utm_term=1

Aha! It looks like they skipped over 1.8.2 and now 1.8.3 is available. It supports sharing vaults!