useful python script for Python 2.7 (you can get version for Python 3.7 here: https://github.com/amorvincitomnia/vtt-to-srt.py)
pip install --user vtt_to_srt
python vtt_to_srt.py pathname [-r]
pathname - a file or directory with files to be converted
-r - walk path recursively
# convert vtt file
from vtt_to_srt import vtt_to_srt
path = '/path/to/file.vtt'
vtt_to_srt(path)
# recursively convert all vtt files in directory
from vtt_to_srt import vtts_to_srt
path = '/path/to/directory'
vtts_to_srt(path, rec = True)