If the FIOS DVR is connected to your DVD Recorder or VHS Recorder with a Coax cable then you would have to turn the recorder to channel 3 or 4. If the connection is with any other type of cable (HDMI, Component, Composite, etc) then you would need to tell the recorder to tune to an auxiliary input.
If you provide details on your DVD Recorder and how it is connected to your Verizon DVR then someone may be able to provide additional instructions.