Shell Command – How to Clone Ownership and Permissions from Another File

I'm not sure Tar can do that. It should be possible, though, to untar both backups (to different directories), then use something along the lines of

cd /mnt/oldbackup ; 
find ./ -exec getfacl {} | setfacl /mnt/newbackup/{} 

as the output of getfacl can be used as stdin for setfacl according to the manpage.

Yes, vim will remove the original file and create a new one to put the new content in.

Your cp && mv -f is the way to go.

Note that when the t bit is set on the directory as it is in your case, it's not enough to have write permission to the directory you also need to be the owner of the file or the directory (as you are).