The data modeling engine IS SSAS. The only major missing pieces are row-level security and defined KPIs. The query language, DAX, and the backing columnstore database are the same engine used in SSAS Tabular.
xVelocity is only a (small) part of SSAS and it has a very limited application compared to OLAP even in the upcoming 2016 edition which greatly enhances the columnar store functionality.