Could you do the same thing with fish, like sardines? They're packed in oil
aren't they, in a can?

You can do that but not in metal containers. It is best to marinate
everything in glass. I recommend that you cure the fish for 1-24 hours in
lemon juice, pour off the lemon juice and then add your vinegar but again,
it is likely to last no more than 2 weeks without getting unappealingly
rank but still very nutritious. I discussed preserving meats for long
periods several issues ago for my survival kit.