Because the configuration of !joomlacomment to work with VM is a little bit trick, I'll post the instructions here so that we can refine them and they will be better understood from everyone.

First make sure to download the plugin from here:
http://compojoom.com/marketplace?page=shop.product_details&flypage=flypage.tpl&product_id=26&category_id=6

*************************Requirements:*********************
1. joomla 1.5
2. PHP5 and MySQL5
3. joomlacomment 4.0 beta2

*************************Instalation:**********************

1. Unzip the com_virtuemart folder and upload it to your website in the following folder:
administrator/components/com_comment/plugin .

2. Open components/com_virtuemart/themes/YOUR_THEME/product_details/YOUR_FLYPAGE.tpl.php and add
the following code at the place you want the comments to appear.

<div style="clear:both">

<?php

require_once(JPATH_SITE."/administrator/components/com_comment/plugin/com_virtuemart/josc_com_virtuemart.php");

?>

</div>

(look at the exampleVMfile folder - I've added the standard virtuemart flypage with the code for joomlacomment)

3.* Now you should be able to create a new virtuemart plugin in the backend of joomlacomment.
extensions->other component settings ->new * ->
from the component dropdown menu choose com_virtuemart and click apply. You should
now see the virtuemart categories in the cat field. You can leave the settings as they are
or specify the categories where the comments should be shown.

* If you don't see the new button, first click on save.
After the page reload the new button should be there.


************************ Be carefull with ******************
1. Find out what the virtuemart database prefix is.
The standard prefix is vm and joomlacomment assumes that you have installed
Virtuemart with that prefix. However if you have installed it with another prefix
you should open the file administrator/components/com_comment/plugin/com_virtuemart/josc_com_virtuemart
and go to line 38:
private $vmp = 'vm'; /* change the value to your VM's database prefix */

change 'vm' to 'YOUR' prefix value .

2. If you want to use the the latest comment module 4.0 . Open the file:
modules/mod_comment/mod_comments.xml find
<param name="component" type="list" default="" label="Component" description="Will display comments of the specified component.">
and add another option for virtuemart like this:
<option value="com_virtuemart">com_virtuemart</option>

3. If you want to use the rss feed option, please make sure that
media/com_comment/rss/com_virtuemart is writable