When you’re speaking of something or someone becoming old, like in “You’re really losing it in your old days”, you can use “sur tes vieux jours”. But in this case you’ve got, this seems to be a friendly conversation, and you remember good times you had with your friend (I hope so). So you should use “Je me souviens du bon vieux temps”, where “le bon vieux temps” refers to an old time you lived with your friend.

Please note that “Le bon vieux temps” has a positive meaning. You cannot use it everytime you speak about something old.

Je me souviens encore vaguement de cette époque / de ce temps là / des jours passés (eventually followed by “où…”, “when…”)

J’ai un vague souvenir de ces moments.

Becareful mes/tes vieux jours doesn’t meaning the same thing. Is old people use that, to speak of the future days (days remaining).

When you speak of memory, you can use moment. It’s usual use.

J’ai aussi une mémoire vague du temps jadis.

“Jadis” is a more poetic term, if you prefer.

I think the translation would be:
J eme souviens encore vaguement des jour passés or it would be better if you said des bons jours.