I would say v-add-mail-account command does not set quota at all.
My question was what is the format of this QUOTA command line parameter. I set it to 1024 and using VestaCP admin panel found it's unlimited, set it to 100M manually. Then I looked into ~/conf/mail/passwd file and found it's 100. OK, now I created a new account
v-add-mail-account usr1 domain.com username 100
and got unlimited again. That's a bug.
P.S. v-change-mail-account-quota (visually) works. Now I will call it just after the previous command.