User blog comment:B214/Question & Answers 5/@comment-98.227.238.52-20150513171226/@comment-1359322-20150514021750

Since the key to unlocking their lost/hidden power is the mutual understanding of the Heavenly Dragons, and Issei managed to unlock the Wyvern when Albion's thought flowed into the BG, Vali might be able to use Ddraig's power too depending on the situation. But that remains to be seen.