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I have good news for you @vending_makina.
Virtualmin natively supports the management of multiple domains by a single user: this is done using sub-servers in combination with a virtual server. If a user has multiple domains, you simply create a virtual server with any one of the domains that the user needs to manage and the other domains can be associated with the corresponding number of sub-servers that you create under that virtual server. It is quite trivial to do so with Virtualmin.
But if you don’t want to use a domain name as the ‘main account’, Vrtualmin will accept, say, vending_makina as a login name and well as the virtual server’s domain name (yeah, I know, vending_makina is not a domain name but Virtualmin will accept it as one - let’s call it a pseudo domain) and you can then use all the domains that you want the user vending_makina to manage as sub-servers of vending_makina pseudo domain.
Both approaches outlined above will also fulfill your requirement of a user being able to login with a single set of credentials to manage multiple domains in Virtualmin.