For this example we'll rebuild the GDAL Precise package.

1. Let's download the files we need from Launchpad. We need to download the .orig.tar.gz, .debian.tar.gz and .dsc.

This step can be done in the Host machine.

$ cd ~/vm/ubuntugis/packages
$ mkdir gdal
$ cd gdal
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0.orig.tar.gz
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0-1~precise1.debian.tar.gz
$ wget https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+files/gdal_1.10.0-1~precise1.dsc

2. With these files downloaded, we simply need to extract the .dsc

This step can be done in the host machine

$ dpkg-source -x gdal_1.10.0-1~precise1.dsc gdal

3. Then you can cd into the folder and update the changelog for a rebuild or make change to the package if you want.

This step should be done in the VM

$ cd /vagrant/packages/gdal/gdal
$ dch -p -D precise -l precise "Launchpad rebuild for Precise"

You can then open debian/changelog and add entries if you have made other changes to the package.

4. With the changelog updated, we can now create the debian source package.

This step should be done in the VM