"When I run vrms, I can't find any non free stuff. But is vrms reliable?"it will detect if a debian package installed on your system is free or not but it wont detect everything
say if you had a non-free program that was stored in your home directory
"As far as freedom is concerned, I was wondering what the difference is between Linux Libre and the Linux kernel delivered by Debian by default."
not entirely sure but i know by default its libre
