Users -> Manage -> and "options" from right corner.
1. There is no option in there. maybe it was in the past, but there is not anymore.
2. "VM Framework Loader during Plugin Updates" this plugin causes the problem. open it in plugin manager, and in the second tab, turn off redirecting. this plugin does two things. one of them is redirecting the registration form. by turning this off, you might affect some other functionality.
3. here is where the problem arises:
in plugins/system/vmLoaderPluginUpdate/vmLoaderPluginUpdate.php
there are two functions (two things that this plugin does, which I mentioned in 2)
the second function causes this problem. it redirects the registration form if the option in com_user options is set to yes. but there is no such option in the com_user. maybe it was in the past.
on line 62 it looks for this option in the com_user to decide to redirect the registration page or not, and because it can't find it, it does the default which is set to yes in this line.
VirtueMart Developers should fix this.