Hi,
I think your command is wrong,you can try vaa3d -x vn2 -f app2  -i    -o   -p  NULL  0 10 1 1 0 0 1 1 0 0 .Since you want to change length_thresh,you can set marker_file  NULL and other parameters default value,the eighth parameter represents length_thresh,so it should be 1.

Best Regards,
Mengya Chen