[SalesForce] in Visualforce Email Template

One approach might be to factor the bulk of your Email Template code (which of course is just VF markup anyway) into a Visualforce Component. Thus enabling you to share it by referencing this component between a Visualforce Page (to enable you to capture it on your attachments) and Email Templates (for when your emailing). For example.

<apex:component access="global">
  <apex:attribute name="targetObject" type="Test__c" description="The context of the outer page / email templates" access="global"/>
  <h1>Congratulations</h1>
  <p>This is your new Component that is shared between your Email Template and Visualforce Page</p>
  <p>This is the name of your record {!targetObject.Name}</p>
</apex:component>

Your email template would look like this...

<messaging:emailTemplate subject="Test" recipientType="User" relatedToType="Test__c">
    <messaging:plainTextEmailBody >
        <c:ShareComponent targetObject="{!relatedTo}"/>
    </messaging:plainTextEmailBody>
</messaging:emailTemplate>

Your VF page would look like this...

<apex:page standardController="Test__c">
    <c:ShareComponent targetObject="{!Test__c}"/>
</apex:page>

Hope this helps!

Note: Note that Visualforce Components when delivered in Managed Packages are none editable, unlike Email Templates. You can work around this though, by copy and pasting the component code into a new one in the subscriber code and changing the template to point to the new component.

Your relatedTo object is the Opportunity, so you should be referencing it as {!relatedTo.field}, not {!Opportunity.field}.