[SalesForce] How to refer the Visual force page in list button to display it in related list

Have you looked into the apex:relatedList tag yet? https://www.salesforce.com/us/developer/docs/pages/Content/pages_compref_relatedList.htm

If this is not a solution for you, please share your full VisualForce page. I think you've only copied the last two lines. You'll have a add a where-clause in your query based on the ID that you pass to the class in VisualForce.

Update:

Please replace your Apex code with:

public with sharing class CompetitorEditExt {

    private List<Competitor__c> comp;
    private Id quoteId;

    public CompetitorEditExt(ApexPages.StandardSetController c) {
        c.setPageSize(10);
        this.quoteId = ApexPages.currentPage().getParameters().get('id');
    }

    public List<Competitor__c> getcomp() {
        comp = [Select Id, Name, Part_Number__c, Price_Offered__c, Product_Series__c, Volume__c, Date_Price_is_valid__c from Competitor__c where CustomQuote__c = :this.quoteId];
        return comp;
    }

    public pageReference save() {
        update comp;
        PageReference ret = new PageReference(ApexPages.currentPage().getParameters().get('retURL'));
        return ret.setRedirect(true);
    }

    public PageReference cancel() {
        PageReference ret = new PageReference(ApexPages.currentPage().getParameters().get('retURL'));
        return ret.setRedirect(true);
    }

}

Now we use the Quote's Id to look for the Competitor records that are related to it. I've also made sure your save() and cancel() methods return to the original Quote.

Your VF page could use a StandardController for Activity and then your VF page's action attribute could execute a method which takes the id from the record and performs a redirect to the flow URL providing that ID as a parameter.

The VF page needs no markup in it other than the <apex:page standardController="Task" extension="yourClass" action="yourMethod" > since no content will be rendered by this page to the end user.

It's just acting as a router to create the URL to the flow and redirect the user.

https://developer.salesforce.com/docs/atlas.en-us.salesforce_vpm_guide.meta/salesforce_vpm_guide/vpm_url_setvar.htm

The VF page

<apex:page StandardController="Task" extensions="activityRouter" action="{!routeToFlow}" >
  <!-- Begin Default Content REMOVE THIS -->
  <h1>Congratulations</h1>
  This is your new Page
  <!-- End Default Content REMOVE THIS -->
</apex:page>

The controller

public with sharing class activityRouter {

    public activityRouter(ApexPages.StandardController controller) {

    }

    public PageReference routeToFlow() {

        // get the parameters from this page
        Map<String, String> pageParams = ApexPages.CurrentPage().getParameters();
        system.debug(pageParams);

        // create a new URL to the flow and append our params to it    
        return new PageReference('/someflowUrl?title=' + pageParams.get('title') + '&what_id=' + pageParams.get('what_id'));
    }

}

Activity "Log a Call" button override

Resulting URL after clicking "Log a Call" button on an Account's Activity History related list https://domain.my.salesforce.com/someflowUrl?title=Call&what_id=001G00000229ESC