Went to check the Boeing CBT,
On the FMA [Thrust Mode, Roll Mode, Pitch Mode]; whichever has SPD, means airspeed is being controlled by that specific mode.
E.g. [THR REF, LNAV, VNAV SPD]; Pitch controls airspeed
E.g. [SPD, LNAV, VNAV PATH]; Thrust constrols airspeed
According to the CBT, you only get SPD in the Thrust Mode, when you are leveling off or in level on the MCP/FMC altitude.
To the question of (assuming we are only looking at descents),
in VNAV PTH the FMC will command pitch to mantain pitch and if required it will command the autothrottle to mantain FMC SPEED
VNAV PTH would most likely give you a Thrust Mode of IDLE/HOLD, as the FMC computed descent profile is based on IDLE thrust.
in VNAV SPD the FMC will retard the throttle levers to IDLE and mantain speed with pitch and will not attempt to stay on path similar to LVL CH
VNAV SPD may go into THR then IDLE/HOLD. VNAV SPD will not maintain the original calculated descent profile which starts from the calculated ToD. It may parallel the profile.
To get VNAV PATH back, you will have to try to increase/decrease your RoD to fall back into the calculated profile, depending whether you are high or low (VNAV profile indicator may assist you).