VLF is "very low frequency" radio waves. RFI is "radio frequency interference". VLF is anything on the radio below 100 kHz -- although technically what is received below 30 kHz is ELF (Extra Low Frequency radio waves).
Are you using a radio, or a microphone? I'm not that up on new technology. I was under the impression you were using some sort of radio receiver. Now that I've re-read your posts it looks like it is an audio microphone.
If you're recording AUDIO, there are a lot of things that could put out low frequencies: distant rumble of trains, trucks, factory noises, etc.