I would like to change “Purchase” button to “En...

You should open following file:

catalog/view/theme/vanityshop/template/product/product.tpl

...and replace following code:

        <div class="quantity"><?php echo $text_qty; ?>
          <input type="text" name="quantity" size="2" value="<?php echo $minimum; ?>" />
          <input type="hidden" name="product_id" size="2" value="<?php echo $product_id; ?>" /></div>
          <input type="button" value="<?php echo $button_cart; ?>" id="button-cart" class="button" />
<div class="clear" />
        <?php if ($minimum > 1) { ?>
        <div class="minimum"><?php echo $text_minimum; ?></div>
        <?php } ?>

...for something like that:

<a href="http://www.diaboliquedesign.com/demo/vanityoc/index.php?route=information/contact" class="button">Enquire Now</a>

Of course you should change this link:

http://www.diaboliquedesign.com/demo/vanityoc/index.php?route=information/contact