Normal VirtueMart 3 action is that clicking on a product image on the category page does take you directly to the full product details page. So it seems that you are using template overrides which do not support that function. In default VM that function can be found in the sublayout at components/com_virtuemart/sublayouts/products.php The relevant code is
<div class="vm-product-media-container">
<a title="<?php echo $product->product_name ?>" href="<?php echo $product->link.$ItemidStr; ?>">
<?php
echo $product->images[0]->displayMediaThumb('class="browseProductImage"', false);
?>
</a>
</div>
Assuming your template developer has actually updated the overrides to VM3 format, then the override being used will be [yourJoomlaTemplate]/html/com_virtuemart/sublayouts/products.php - so that is the file you will need to edit.
Or get you template developer to fix it.