In essence a Variable camber is "slotted", a Kreuger is not.
You can have slotted Krueger Flaps too. The real difference is that the Variable Camber Flaps change camber during extension, the flexible panel bending around the four bar linkage underneath. Essentially they are Krueger Flaps that have a curved surface. When retracted the VCF is flush with the relatively flat wing lower surface, but when extended the panel is cambered like a LE Slat.
Look at the photographs and video and you can see this happening. The VCFs have a curved surface, but the Kruegers remain flat.
Both VCF and Kruegers change the camber of the wing too of course.
I haven't been able to find a convincing answer to what kreuger flaps actually do
In operation, Krueger Flaps work like drooping leading edge flaps, but without a lower surface. The lower surface airflow detaches at the leading edge of the Krueger and re-attaches on the wing lower surface, forming a separation bubble. Therefore a lower surface is not really missed. Kruegers also make good speedbrakes.
They increase wing area and camber in a crude but effective way. With the flap extended the stall angle of attack is increased, just like any leading edge device.