hi there
VDR's are not quite the correct term
in a degaussing circuit of a CRT type TV the device is called a PTC
These devices, sometimes 2 pin, sometimes 3 pin, start at low resistance and over several seconds of power applied go very high resistance
This allows mains current to briefly flow through the degaussing coil and demagnetise the screen each time the TV is turned on.
The device in a power supply between the mains input and the rectifier is called a MOV ... Metal Oxide Varistor
It operates very differently to a PTC. A MOV will measure low resistance and will allow current to flow indefinately, as long as the Voltage is within its manufacturered tolerance range.
When the Voltage spikes ( say with a surge) this will also cause the current to spike. This causes the MOV's resistance to increase greatly and limit current flow to the rest of the power supply from the mains supply.
MOV's come in a wide range of voltage and current specifications, so therefore you need to figure out how much current will normally be flowing ( including any peak demands) and select a MOV specification to suit
cheers
Dave