If I remember correctly, the performance calculations assume level flight. I did fly for one operator who did not trust this new failed TOGA mumbo jumbo and used V/S to clean up on a normal day. Engine failure, of course they used ALT HLD.
Boeing TOGA FD will give level flight or up to 200fpm ROC. It will not allow descent. Hot high, heavy weight, max ATO thrust etc. will give 0'fpm ROC. Any excess thrust will allow a breath of a climb.