There is a pitch control toggle on the Pedestal.
If you engage VS on the MCP, it will engage the AP and maintain your current VS rate.
You can then use the Pitch control to adjust this.
Or, if stable with AP Altitude ON... change the value in the Altitude window, then hit VS on the MCP, and adjust the VS rate using the pitch control.
It will only work if you change the altitude.
I use a button box with small trim wheels / Streamdeck, which makes this easier... but I'm sure you can find the relevant commands and assign a pair of buttons/rocker switch on one of your controllers ?
(Don't forget VS should only really be used for small changes in altitude. IAS is the safest way)
The Desc mode referred to above is not really relevant... it just tells you the minimum N2 (?) required for electrical systems etc to work properly.