Come again Felix ?
If you have VNAV SPD you most certainly can be below the path....Its called SPD Intervention and it will get lower and lower below the path until you do something else.
VNAV ALT ?? You only get that if you select VNAV and capture an MCP entered altitude that is not in the programmed VNAV profile as a hard altitude. It will never appear just because you are low !
VNAV will not let you go below profile when APPROACH LOGIC engages and in that case VNAV SPD will be replaced by VNAVPATH