Will the Vanguard and Sabre be in the mini PU, or [added] afterwards as ships are built and become flight ready for Arena Commander?

I imagine they'll be added to the PU. But the Sabre just closed its concept sale this week, so it's quite a ways away from that.

It can be a long road from concept to complete just because these ships are really, really complex. I'm sure some of the people who like digging for XMLs have seen just how weird some of our part lists and just how things get hooked up can be.