What I said previously applies even more so in the event of a TCAS resolution. With V/S engaged, the flight management is in a 'speed' mode and therefore the vertical profile remains constant. In an open mode such as FLCH or CLB, constant thrust is played against pitch to (paradoxically) maintain a climb airspeed. Such a mode can lead to short term variations in vertical speed/profile which deviate from the TCAS computed solution.
Either way V/S 1500 is a sensible compromise.