Yes, your class looks functional.

Here is what your example looks like with some tests:

global with sharing class myRewriter implements Site.UrlRewriter {

    String VIP_PAGE = '/VIP/';
    String VIP_VISUALFORCE_PAGE = '/VIP?id=';

    global PageReference mapRequestUrl(PageReference myFriendlyUrl){
        String url = myFriendlyUrl.getUrl();
        if(url.startsWith(VIP_PAGE)){
            String name = url.substring(VIP_PAGE.length(), url.length());
    
            Lead ld = [select id from Lead where XUniqueSearchId__c =: name LIMIT 1];
            return new PageReference(VIP_VISUALFORCE_PAGE + ld.id);
        }
        return null;
    }

    global List<PageReference> generateUrlFor(List<PageReference> mySalesforceUrls){
        return null;
    }
    
    @isTest
    static void testGenerateUrlFor() {
        myRewriter rewriter = new myRewriter();
        System.assert(rewriter.generateUrlFor(null) == null);
    }
    
    @isTest
    static void testMapUrl() {
        Lead testLead = new Lead(XUniqueSearchId__c = 'testId', lastName='Guest', Company='Salesforce');
        Id testId = Database.insert(testLead).id;
        
        
        myRewriter rewriter = new myRewriter();
        String actualResult = rewriter.mapRequestUrl(new PageReference('/VIP/testId')).getUrl();
        
        String expectedResult = '/VIP?id=' + testId;
        //System.debug(actualResult);
        System.assert(actualResult == expectedResult);
    }
    
    @isTest
    static void testMapNonVipUrl() {

        
        
        myRewriter rewriter = new myRewriter();
        PageReference actualResult = rewriter.mapRequestUrl(new PageReference('/testUrl'));
        
        String expectedResult = '/testUrl';
        System.assert(actualResult == null);
    }

}