Setting up Pressflow 6 with Vagrant VM
After trying quite a few things, we ended up putting together a shell script to do the provisioning. Most of it is specific to our application, but here are the relevant excerpts for anyone who might want to try this:

(there is probably some configuration necessary that this script omits)

#!/bin/sh

cd ~

echo "Beginning fresh install..."
sudo apt-get update

echo "Installing base utilities..."
sudo apt-get -qq -y install zip unzip

# Install LAMP stack
echo "Installing LAMP stack..."
sudo apt-get -qq -y install tasksel &amp;&amp; sudo tasksel install lamp-server
sudo apt-get -qq -y install php5-gd php-pear php5-xdebug php5-dev php5-curl
sudo a2enmod rewrite # Enable apache rewrite module

# Install pressflow if necessary
echo "Downloading and installing pressflow..."
wget -qNO /tmp/pressflow.tar.gz https://github.com/pressflow/6/archive/pressflow-6.26.109.tar.gz
sudo rm -rf /tmp/pressflow &amp;&amp; mkdir /tmp/pressflow
tar -xzf /tmp/pressflow.tar.gz -C /tmp/pressflow
sudo rm -rf /tmp/pressflow/6-pressflow-6.26.109/sites/all
rsync -a /tmp/pressflow/6-pressflow-6.26.109/* ~/yoursitepath # !! Your Drupal directory here.
sudo rm -rf /tmp/pressflow /tmp/pressflow.tar.gz

# Update php.ini
echo "Updating PHP config options..."
PHPINILOC="/etc/php5/apache2/php.ini"
sudo sed -i 's/memory_limit = 128M/memory_limit = 1024M/g' $PHPINILOC
sudo sed -i '/error_reporting = E_ALL.*/c\error_reporting = E_ALL &amp; ~E_NOTICE &amp; ~E_STRICT' $PHPINILOC

# Create database schemas
echo "Creating databases..."
mysql -uroot -e"CREATE DATABASE IF NOT EXISTS ourfa5_drupal; CREATE DATABASE IF NOT EXISTS stats;"

# Here, you'll probably need to download any default tables needed by your
#   application, update /etc/hosts, etc.. This portion of the script is
#   specific to our application so I've left it out.

# Restart apache for good measure
sudo service apache2 restart

sudo apt-get upgrade