Good question. We just ran a plane up doing an overspeed check. I'll have to go back and look again. I don't remember the trend vector being there. It would make sense that no trend vector could appear without inertial data.
Here's what the SDS section 34-21-00-010 has to say:
"DEU
The DEUs use this IR data from the ADIRUs to show information on the display units:
Present position, Track angle, Magnetic heading, Wind direction, True heading, Ground speed, Drift angle, Flight path angle,
Flight path acceleration,
Pitch angle, Roll angle, Body pitch rate, Body roll rate, Body yaw rate, Body longitudinal acceleration, Body lateral acceleration, Body normal acceleration, Track angle rate, Pitch attitude rate, Roll attitude rate, Inertial altitude, Along track horizontal acceleration, Cross track horizontal acceleration, Vertical acceleration, Inertial vertical speed, N-S velocity, E-W velocity."
and here's SDS 31-62-00-301:
"Speed Trend Vector
The speed trend vector shows the predicted airspeed in 10 seconds based on current acceleration or deceleration. This vector shows when the speed trend is more than 4.5 knots."
Sounds like a combination of elements to me...