According to the Bendix manual, there are two ways to engage VS mode.
First is ‘Vertical Speed Preselect’. Adjust the VS preselect value in the KAS 297B selector and press the ENG button while the value is displayed. Altitude Hold, if engaged, will be canceled and the system will capture the commanded vertical speed. If a change in vertical speed is desired, the vertical trim rocker switch (UP/DOWN) fom the AP control panel can be used.
Second is ‘Vertical Speed Sync’. By just pushing the ENG button while the KAS 297B is displaying the preset altitude, the system should maintain the current vertical speed. Upon enganging, the KAS 297B should display the vertical speed for 2 seconds.
I never used VS mode. So you should try this procedures yourself. Can you report which of the two methods work?