I did not see a specific VLF applicable area, so I figured this is a good starting point. This balloon will be transmitting on 14.2 kHz, and they are looking for reception reports. (There will be an APRS beacon, too.)
VLF Balloon with 210m long antenna launches Sept 12 | Amateur Radio News
A high-altitude balloon experiment, launched by Warsaw University of Technology, is planned to lift off September 12, carrying a VLF 210-m-long fully-airborne antenna system, transmitting on 14.2 kHz
www.southgatearc.org