Hi,
If you use VirtueMart on your Joomla website, then your customer have normal Joomla user accounts.
And when you install HikaShop, it synchronizes the all the Joomla user accounts.
So what you're talking about is not possible, since even before the import from VirtueMart, you should already have all the Joomla user accounts and they should already all be synchronized with HikaShop.
So I suppose that you must be trying to integrate data from a VirtueMart from another website, without first importing the Joomla users from the other website.
Before the VirtueMart import, run an import of the Joomla users (you'll find several extensions on the Joomla extensions directory in order to do that), and then install HikaShop on the website (so that the synchronization of the users is run). That way, you should not get the problem you're talking about.