Destroy is to me the problematic part of Nightblood's command, after all a regular shardblade only severs the soul, while Nightblood destroys it and on all 3 realms just to top it off! For me this is probably an big part of the difference between Nightblood and Vivenna's blade. Another big reason for why it is Vivenna is that Vasher in Warbreaker did not teach Siri or anyone else how to fight only Vivenna, he traveled with Vivenna afterwards, and Azure where given the same education as Aidolin and the one that Kaladin started. Azure have the royal hair, did any of you note that she where able to take color for her hair in place of anything on or around her when she awakens objects! Kaladin notes her hair shifts to Gray when she where ready for an fight with her cloak around her arm the same one that where hanging down in a wrong way! Considering hair isn't alive only the roots are, this is consistent with awakening lore! Question did she ever call herself an bounty hunter or state she where there to capture the criminals? If I remember correctly she stated she where hunting an criminal and an associate of this criminal. It is totally possible that Vasher went to Rosar to get away from everyone and Vivinna is trying to find him again. It is totally believable that Vasher ran away while Vivinna where out of the house so to say.