On Thu, 31 Mar 2016 19:56:04 +0530, use experience stated:
>> unix_listener auth-userdb {
>>
>> mode = 0777
>>
>> }
Try adding:
user = vmail
group = vmail
Why are you using mode = 0777? I never use more than 0666. Usually just
0660 and all works well. Try to include the "group" in your definitions.
Restart dovecot and see what happens.