If my opponent used Banette ex's "Everlasting Darkness" attack on their last turn, can I attach Vitality Band to my Pokemon on my turn?
Yes, Vitality Band is a Pokemon Tool card, not an Item card.
Source: Scarlet & Violet FAQ; TPCi Rules Team (2023-03-30)

Relevant cards

Note: All applicable cards may not be listed below.
SV1_88
SV1_88
SV1_197