Tłumaczenie - Angielski-Łacina - Guardian angel my guardian dear, to ...

Angele qui custos mei es, care custos mei
--> Angele custos mi, custos care mi

They are supposed to be in the vocativus, aren't they?

I've chosen "mei" = genitive, pronoun "ego" ("guardian of me" because I've found that the original prayer has "custos mei" and not "custos meus".

http://digilander.libero.it/pietates/preghiere%20in%20latino.htm#ANGELE%20DEI

Yes, of course, it can be "mei custos", dear. It means exactly "guardian of me". However, if we wanted to say simply "my guardian" in the Nominative it would be "custos meus" in the Vocative - "custos mi". Both forms "mei custos" and "custos mi" are correct.

But the English source reads: "Guardian angel my guardian dear" not "Angel, who are my guardian..." (Angele qui custos mei es...)

That's why I suggested: "Angele custos mi, custos care mi"

Of course it can be also "Angele mei custos , mei custos care", but your version just contains something more what I can't find in the source above: ("qui custos mei es" ).

Ok, dear, I'll choose your second option.