if admin enabled the Subtotal, the same should be enabled for vendor. For this follow this :
Override this template by copying it to yourtheme/wcmp-pdf-invoices/vendor-pdf-invoices-settings.php
<?php $setting = get_option( 'wcmp_pdf_invoices_settings_name', true ); ?>
<div class="form-group">
<label class="control-label col-sm-3"><?php _e('Subtotal', 'wcmp-pdf_invoices'); ?></label>
<div class="col-md-6 col-sm-9">
<input type="checkbox" id="is_subtotal_vendor" name="is_subtotal_vendor" class="checkbox" value="Enable" <?php if ($setting['is_subtotal_admin'] != '') echo 'checked="checked"'; ?>>
</div>
</div>
