I'm trying to do just that. A separate install for vdr and xbmc. No integration for me now, since the plugins functionality is lost.
A few methods are described here - http://www.xbmc.org/forum/showthread.php?t=47560.
with that scrip you can run vdr or xbmc independently
!/bin/bash export LANG=ru_RU.UTF-8 export LC_ALL=ru_RU.UTF-8 /usr/src/VDR/vdr --lirc --localedir=/usr/src/VDR/locale -s /etc/vdr/vdrpoweroff.sh -v /video -c /etc/vdr -u root -L /usr/src/VDR/PLUGINS/lib -P"xine -r" &
while [ 1 -eq 1 ]; do
svdrpsend.pl remo on &&
xine -f --post vdr_video --post vdr_audio --post upmix_mono --verbose=2 -V vdpau -A alsa "vdr:/tmp/vdr-xine/stream#demux:mpeg_pes"
svdrpsend.pl remo off && xbmc -fs -l
done