I expected compiling of PHP 7 to be more difficult considering it was still an RC1 release and Ubuntu Vivid is not an LTS release. However, it turns out compilation of PHP 7 actually requires more new packages so it was easier than on 14.04 Utopic.
From what I could tell you only need to have two packages installed: build-essential and libxml2-dev. I might have already had extra packages which were required because this is a fully setup desktop for development, so it already includes nginx & hhvm.
The steps to follow are pretty slim and should hopefully be sufficient for most:
sudo apt-get install build-essential
sudo apt-get install libxml2-dev
tar xzf php-7.0.0RC1.tar.gz
sudo make install
And that should do it.