Calculating sentences / paragraphs vectors can be done in many ways. For example a simple method  is to average all the words vectors and retrieve a single vector for the entire piece of text, of course this forces you to have pre-calculated word embeddings (I already wrote about it here).

In this post I will show a different approach that uses an AutoEncoder

The aim of an AutoEncoder is to learn a representation (encoding) for a set of data, typically for dimensionality reduction, by training the network to ignore signal “noise”.