Hi Colin
It due to the conflict in style between Virtuemart and the administrator template Atum of Joomla (again the error from VM 4)
I updated this file: /administrator/components/com_virtuemart/assets/css/admin_ui.css and change the header color so it can look better
#header { background: rgb(31, 48, 71) !important; border-bottom: 1px solid #D3D3D3;}