Pitch modes TOGA/FLCH SPD/VNAV SPD are all speed controlled by elevator,
One push of the TOGA switches gives you THR as you said which aims initially for 2000fpm roc it doesn't chase it there's a transitional phase where things are blended, it depends on the pitch rate and acceleration for when the algorithms/computer think it's added sufficient thrust to achieve the 2000fpm. If the rotation is very slow you'll end up with over 2000fpm. Way beyond the scope of flying. Eventually you'll be in TOGA with speed on elevator and the resultant roc a reflection of the performance during the rotation.
Selecting VNAV at 1000'aal will give you THR REF (CLB thrust) & VNAV SPD it's basically exactly the same, however it also closes the speed window thereby allowing the A/C to accelerate, so the A/C will pitch down or command a pitch down as it's now at the wrong commanded speed.
Opening the speed window in TOGA and winding the speed up would do the same thing except the thrust would stay fixed in THR at the original thrust it locked onto while attempting to achieve 2000fpm roc.
Last edited by SMOC; 12th Apr 2014 at 01:13.