On a domestic-only flight (no intl'l segments) -- you can get the last seat.

Any reservation containing an international segment, the seats are capacity controlled for ALL flight segments (including the domestic connections).

For example -- LGA-ORD-LGA -> VIPOW will upgrade you to the next class of service as long as a seat is available (i.e. there is a revenue seat available).

JFK-LHR-JFK -- VIPOW is capacity controlled and award seats must be available.

LGA-ORD-FCO-ORD-DFW-LGA -- VIPOW is capacity controlled on all segments including domestic segments(specifically LGA-ORD, ORD-DFW and DFW-LGA).