I have also trouble with VNAV. When I adjust speed and FL for each planned waypoint , the plane (B737-800 standard) will not follow VNAV. If I switch to V/S, then the buttons LCHG and N1 will be activated and the plane climbs to the altitude which is adjusted at the MCP (?) and not to the level of the FMS. Take care, if the altitude value of the MCP is lower than the FMS value, this is correct. If the value of the MCP is higher and ALT HOLD ist not active, VNAV shal be the master, but this fails during my tests.
If You reach the waypoint, where the descent has to start, You will see at least the same result, Nothing will happen. Switching V/S on, the plane will descent and follows VNAV with various vertical sinkrates due to the descending angle.
May be ,You have to look, that VNAV, and LCHG ist active at this moment. So finally You will reach the altitude level which You have adjusted at the MCP. It helps a little bit, to reach the initial point for the ILS, which You will expect for Your target airport
I have checked this several times, by using a short flight plan from EDDV/27L via NIE7F, NIE, IDEKO, ESTAD, OSTOR, OLDIP, CF32, EDXW.
I ´ll thing, VNAV is a little bit buggy? Hoping of the experience of other flight simmers who can help us.