Would you say a leaf cell or a leaf vessel or a leaf tissue or something else?!

Its more logical to say a leaf tissue because of the presence of several tissues inside it which are performing their designated functions.

Explanation:

If we look at the cross section of a leaf, the leaf tissue looks having several tissues. There are vascular bundles in the midrib that are xylem and phloem. Then there are leaf parenchyma which contains the chloroplasts.

They are palisade parenchyma on upper layer and spongy parenchyma. Then there are leaf epidermis on both upper and lower surfaces of leaf. So because of the presence of different tissues, they are collectively called as leaf tissue.