Simplesmente aptitude download
it:
$ ls vim*.deb
ls: vim*.deb: No such file or directory
$ aptitude download vim
[...]
Fetched 835kB in 0s (2933kB/s)
$ ls vim*.deb
vim_1%3a7.0-122+1etch5_amd64.deb
$ id
uid=1000(earl) gid=100(users) groups=100(users)