If you want to get the current selected data source record you can use dataSource_ds.cursor() method
See code below
public void executeQuery()
{
super();
if(Al0VendorActivity_ds.cursor().RecId != 0)
{
QueryBuildDataSource productCategory_ds = this.query().dataSourceTable(tableNum(Al0VendorSearchCriteria));
productCategory_ds.addRange(FieldNum(Al0VendorSearchCriteria, VendorActivityRefRecId)).value(int642Str(Al0VendorActivity_ds.cursor().RecId));
}
}