How to store/retrieve multiple ansible-vault passwords within/from a single file
I totally misunderstood you. Here is a new attempt to actually answer your question. :)

Create a passwords file:

root@2ca77340d571:/# cat .passvaults
dev my_dev_pass
test my_test_pass
prod my_prod_pas


Create a file to encrypt:

root@2ca77340d571:/# cat extravaulty.yml
chocolate:banana


Encrypt the file:

root@2ca77340d571:/# ansible-vault encrypt --vault-id=dev@.passvaults extravaulty.yml
Encryption successful


View the raw file:

root@2ca77340d571:/# cat extravaulty.yml
$ANSIBLE_VAULT;1.2;AES256;dev
33336336336238326137393764316333623231336238323931306166626434653164326330656566
3561356333343435663538623661363661373461356461300a663361666331323031623530343930
62633230636566643339663637386438336539383162346634393031633165333033396238363163
6134383233346538330a326266626261346638303738313862656337396237343231326231656139
34376334663664343164303462356431306539316232323761386164643735376330

and the decrypted file:
root@2ca77340d571:/# ansible-vault view --vault-id=dev@.passvaults extravaulty.yml
chocolate:banana

As I understand it, when you use --vault-id=dev@.passvaults you are telling it to use dev as a hint (not the vault password) and .passvaults as the file where the encrypt/decrypt password is.
This means that you can also do this:
root@2ca77340d571:/# ansible-vault view --vault-id=prod@.passvaults extravaulty.yml
chocolate:banana

Or this
root@2ca77340d571:/# ansible-vault view --vault-id=.passvaults extravaulty.yml
chocolate:banana

Which is not intuitive to me.
Does this help?