I didn't realise they had ordered any A319 aircraft so I headed over to Wikipedia and apparently they only have 1 of them on order. Is this correct? If so, why only order 1 of them?
VY doesn't order a/c, their fleet is managed through IB, which has given them a good number of their most recent A32Xfamily a/c lately. Most are painted directly in VY colors at the Airbus FAL.
As from a crew training point of view there is no difference between an A319 & A320, and the fleet maintenance is shared with IB, there is no problem in having just one a/c, although it is likely that more will come.