During the Nara period, ヰ was pronounced as [wi] and イ as [i]. In the Man'yōgana, there were characters to represent ヰ (e.g. 井, 位, 爲, 猪, 謂, 藍) and イ (e.g. 已, 五, 以, 伊, 怡, 射, 移, 異); no characters for one could be used to pronounce the other. The labial glides ク [kʷi] and グ [gʷi] also existed (though in those days small script kana were not used for glides), and were distinct from キ [ki] and ギ [gi].
