777 stuff
THR REF vs THR
Autothrottle is commanded to a limit manually set by you in the FMC vs Autothrottle is commanded to maintain a vertical speed (as in the case of a G/A)
VNAV can be armed and disarmed only when not engaged by pressing VNAV again unless arming the APP
So VNAV can only be disengaged in your case by selection of another pitch mode TO/GA, FLCH SPD, V/S, FPA, ALT or G/S (selected by arming APP)
When the aircraft captures the MCP selected altitude the Pitch FMA will become ALT and the ALT Hold light on the MCP will come on.