There is a custom registration and login module, but no user hook plugin, so you *must* use the virtuemart user manager, login, and registration modules. Using the core joomla user manager, or allowing core registration results in the user database getting out of sync.
There is no built in user sync capability post-installation to rectify this, you must handle the problem manually in the database.
The documentation is extremely poor. Fields are described similarly to how the Joomla API auto documentation works, e.g. the setting to make a user field required simply states "sets this field as required or not" without telling you if it's required for registration, checkout, or both. I can tell you from experience, it's not the second or third option, which causes problems.
Help requests in the forums are handled one of three ways, in order of occurrence:
1. Completely ignored.
2. Answered by a user with an useless but honest attempt to help, e.g. "is it plugged in?"
3. A suggestion to read the source, which would be fine if it wasn't such a poorly formatted, undocumented disaster.
The "API" is nearly the worst documented one I have encountered in almost twenty years as a professional developer.
The template system is atrocious, and it is nearly impossible to modify the simplest of layout items without doing core hacking of virtuemart. Once you find the appropriate file, you'll soon learn that the slightest modification of the code results in a cascade effect of destruction throughout the cart.
For example, adjusting the width of the div holding the registration information affects both the signup/register AND the checkout pages, even though they have entirely different layouts.
I'm not certain why it has such a high rating, but I suspect it's because people who are simply using it as a cart, with none of the advanced features, on a basic Joomla site, haven't seen any or many of these problems.
The truth is, for a site that will be even slightly customized beyond having a new template, virtuemart is a horrible choice. You will regret it every day you are forced to use it.
So, don't. Find another solution, or build one.