using bittorrent for backup of personal files

[Rob Owens <rowens@ptd.net>]

I'm considering using bittorrent to back up large files such as pictures
and home movies.  I am the admin for several of my family members'
computers.  The idea would be to back up my files onto their machines,
then eventually back their stuff up in the same manner, resulting in 
several off-site backups for each of us.

I already use BackupPC successfully for offsite backups between family
members.  Very large files are sometimes a problem, however, which is
why I'm considering bittorrent to supplement BackupPC.

I want to keep this data private.  What are my options, besides a VPN?
I hesitate to use a public torrent even on encrypted data, because the
computers of tomorrow may easily crack today's encryption using brute 
force.

Has anybody used bittorrent over VPN?  Searching for those terms just
gives me hits on VPN services that can be used to keep the MPAA and RIAA
out of your face.

My data is sorted in directories by year.  If I make torrents for each
year, most of the data will be static.  But how should I handle the
current year's data?  Can I update the torrent file without forcing a
re-download of all the current year's data?

Thanks for any advice you may have.

-Rob