PHP notice unserialize(): Unexpected end of serialized data
Update:

Craft 2.6.2957 added support for PHP 7.1



There's a lot going on here, but 1st thing is 1st.


  The vagrant box is running PHP 7.1


Craft 2 (and the current latest version of the Yii framework - 1.1.17) are not yet compatible with PHP 7.1.

When the next Yii 1 release is out that addresses the framework compatibility issues, we'll get Craft 2 up and running on PHP 7.1.

Until then, you'll want to use PHP 7.0.x.