On our 744's we retain ETA and fuel predictions after a vector.
Subjectively, it seems to calculate the track miles and hence times and fuels by calculating a turn from the 'vector' leg when abeam the point furthest out on the inbound course for the approach to that point itself. It does not draw in that track however.
Seems to get the predictions pretty accurate too - thinking here about the arrival to JNB 03 from the north as an example.