Since you're new to VAM, I'll give you some explanation, which might or might not be redundant.
The body of a Person Atom in VAM has nodes, or controls, which is essentially how we can position the body parts in a scene. In desktop mode, you can press T to view the active nodes of a Person Atom, which will be seen as green squares.
They have several position states, like "Control", "Hold", "Off", among other ones. The reason you can move your head and hands normally is because they're set to Control, and then you possess and control them with your trackers, while the lower body nodes (feet) are probably set to Off. Some creators use this kind of strategy so you're able to move around inside the room, which would not be possible if the feet were also set to Control and you had no way to control them with trackers.
If you don't have trackers for the the feet, this is unfortunately gonna happen, because the Off setting makes it so they completely respond to your head and hands movement. What you can do to minimize the floppy effect is to use the "Comply" setting for the feet controls. Comply allows a node to hold it's position as long as nearby body parts don't move too much. You can adjust thresholds for this, from completely floppy (near zero spring and damper threshold values) all the way to a stiff response (high threshold values).