Hi Zoee,
At the moment we are only capable to give vitamins to our own kitties.
All Love vitamins bought at the store are no transfer. However, in the past we have had Limited Edition kitties (like the Halloweens or the Christmas Collection) that included a Love Vitamin and/or a Love Drop in the pack. These packs and all it's contents are transfer as long as they are not unpacked and individually rezzed (the new owner rights apply in the moment you rezz the Love Vita/Drop so if you pick them up again to your inventory, they will be no trans from that moment on)
However, if you already bought the Vitamins, you can ask your friend to pass you their kitty, you give them the vitamins and then you can pass it back, all this providing that your friend's kitty is one that can be transferable (starters bought at the main store or obtained as gifts from KittyCats are usually no trans)
I hope this helps