Let's say you want to maintain 270kt till FL70. Then reduce to 250 kt ,followed by 210 at REDGO.
You should select (in descent page ) , descent speed 270 ,speed restriction 250kt/FL70 ,and in legs page REDGO 210/3000 . This way you'll create a vertical profile and enable the FMS to create a path.If the FMS will think something is not right ,you'll get a message ,like "steep descenty after XX ' ,for ex.
To be more accurate,don't forget to fill the forecast page ,at least with some estimated wind,and qnh.
And don't believe the ones telling you the VNAV it's never working ,it's just not knowing to make use of it. It's a computer, it has to have corect in-data to give correct output.