The file did not exist because it is your responsibility to create it.

If you understand the T3 framework and the Elycion template, it's in the documentation on how to work with custom code.
Otherwise, there is no need to put in cusom.less, but there is a file com_virtuemart.less in the extra folder where you can edit it straight away and you don't need to set the !important flag - that's just a last ditch brake and I'm breaking our coders' hands for that kind of work.
Again - it's all in the documentation or just a simple click through the folders after installation and look at the template settings
And there is also paid support - where they will be happy to help you, because elycion is a paid template and then you are entitled to a support ticket.