If you want to define template (tpl) for certain product or category this feature will be helpful you. Tested on OpenCart 1.5.X and vQmod 2.1.6.

So... you need vQmod...

HOW TO USE:
1. Copy template_for_products_in_category.xml to /vqmod/xml
2. Create file like product-category-ХХХ.tpl where XXX is category ID... as in WP... Note: this file should be located in /catalog/your_template_name/template/product/
3. Have fun! All the products which belong to category XXX will use product-category-ХХХ.tpl as a template. (Btw, if product belong to multiple categories the feature will take only the smallest ID value)

If you want to use special template for certain category page you should create file category-XXX.tpl where XXX is the category ID.

P.S. Best way to say "thx" - to rate the plugin and share your experience in comments.