The MSSQL could be installed to the machine during its provisioning - the user will be asked a question whether he/she wants to have it.
vagrant ssh- login to VM;
sudo su - postgres- change system user to database manager;
psql- login to Postgres SQL shell;
\l- see the list of databases;
\du- see the list of users;
Configure a database for Drupal.
CREATE DATABASE drupal;
ALTER DATABASE drupal SET bytea_output = 'escape';
ALTER USER root WITH PASSWORD 'root';
GRANT ALL ON DATABASE drupal TO root;