A Vertical Machining Center (VMC) is a machining center where the spindle is oriented vertically. The spindle refers to the milling spindle which holds and rotates the cutting tool. This differs from a turning spindle which holds and rotates the workpiece.
The image below shows a typical vertical machining center - the DNM 5700 from DN Solutions. The photo shows the machine with its safety door open, revealing the workpiece table and the vertically oriented milling spindle above it.
Image: DN Solutions DNM 5700 Vertical Machining Center