I was able to convert a date value into a integer data type in VDP by using any of the following expressions:
cast('int',concat(getyear(<field_name>), getMonth(<field_name>), getday(<field_name>)))
cast('int',formatdate('yyyyMMdd',<field_name>)) from <view_name>
Click on the Help menu > Functions List where you can find lots of predefined functions related to date available in VDP. Also please refer Advanced VQL guide where it explains with example on how to use these functions.
Hope this helps.