first of all in my opinion you got strange flightlevels
Basically VNAV is the vertical path that's put in the fmc.
If you have any altitude restrictions while you are climbing and the mcp alt is set higher, if you then press alt intervent the altitude restriction is deleted.
If you are in cruise and want to descend in vnav you have to put your new altitude in the fmc which you can do manually or by setting the lower altitude in the mcp and pres alt intervent.
I'm not sure anymore if it's the same when you already entered the approach in the fmc. Ours says reset mcp alt when we reach our TOD