We are a starting web hosting company. We are testing Vesta and have found what is obviously a bug in your control panel. This my not be the most appropriate place to report it, but I could not find a proper place in your forums, so I picked what I thought was the best place. If I choose wrong, I am sorry.
You see, one of our domains is shoppingonlinepy.com. Vesta automatically assigns it the prefix "shoppingonlinepy_" for all databases names and users.
The problem is that the assigned prefix is already longer than 16 characters (17 in this case, which just for 1 character names makes it 18 characters long), so MySQL keeps on rejecting it.
I know it is a MySQL bug really, but you could as a workaround provide an editable prefix... That can only be edited when the user has no databases or users.
On this same note, another case in which such a prefix would be useful is when you own different domains, say "shoppingonlinepy.com" and "shoppingonlinepy.net". What would the database prefix be in these cases? Note that I have not tested this, but the exact same solution could apply.
As a work around, I have created my users using phpMyAdmin, logged in as the root MySQL user. So I am not in any sort of hurry... But by doing so, I have extracted the domain's database component from Vesta's control. So it would be nice if you provide a fix.