The secure and non-secure items warning is not a problem of VM and so there's little, if any, VM can do. (By the way putting a http link in a secure page will not cause the warning. The warning only occurs when the object is downloaded with the page.)
Judging from the problems I've fixed so far, most of the time the non-secure item is due to modules and templates outside of VM. If you use a third party modules/mambots/templates that has a non-secure item, the warning will come up. As obvious, VM cannot handle problems caused by these items.
Another source of problems is caused by custom VM modules and templates. By hard coding http in your template, eg, the secure and non-secure warning will come up when using https. Again, this is nothing VM can do.
To sum up, the secure and non-secure warning is caused by the modular and flexible structure of Joomla and Virtuemart. And that's the beauty we all treasure. The development teams cannot and should not guarantee every customized item is without problem. The end users/developers themselves should take up that responsibility.
Virtuemart has its weaknesses and rooms for improvements. But please don't blame VM for every problem you encounter.