Yes, the failure is in my test class.  The error is on Line 74: Failure Message: "System.AssertException: Assertion Failed: Expected: 001234, Actual: null", Failure Stack Trace: "Class.TestVDUpdateAccountNumber.testBulkInsert: line 74, column 21 External entry point". Here is my code.

@isTest
private class TestVDUpdateAccountNumber {
    static testMethod void testInsert() {
        List<VDDocs__c> vdDocs = new List<VDDocs__c>();

        // create user to run the test as
        User u = [select id from user where name = 'Lindsay Kelly'];
        
        // create test account
        Account testAccount = new Account(Name='Wendy Test Company Name', AccountNumber = '001234', Type = 'Client');
        insert testAccount;
        
        // get testAccount id
        String AccountID = testAccount.id;
        
        // test single insert
        VDDocs__c v = new VDDocs__c(
                Name = 'wendy test document' ,
                OwnerID = u.id,
                Account__c = AccountID,
                Work_Order_ID__c = '123456');
        
        Test.startTest();
                
        System.runAs(u) {       
            insert v;
        }
        Test.stopTest();
                
        // query for inserted doc
        list <VDDocs__c> insertedDocs = [SELECT Work_Order_ID__c, Account_Number__c FROM VDDocs__c WHERE Account__c = :AccountID];
                    
        for(VDDocs__c v1: insertedDocs){
            system.assertEquals('001234', v1.Account_Number__c);                   
        }
    }
     static testMethod void testBulkInsert() {
        List<VDDocs__c> vdDocs = new List<VDDocs__c>();
        List<account> accounts = new List<account>();
        
      // create user to run the test as
      User u = [select id from user where name = 'Lindsay Kelly'];   
      
      // create test account
      Account testAccount = new Account(Name='Wendy Test Company Name', AccountNumber = '001234', Type = 'Client');
      insert testAccount;
        
        // get testAccount id
      String AccountID = testAccount.id;
        
        // add 200 versadoc documents to the list to be inserted
      for (Integer i=0;i<200;i++) {
            
            VDDocs__c v = new VDDocs__c(
                Name = 'wendy test document' +i,
                OwnerID = u.id,
                Account__c = AccountID,
                Work_Order_ID__c = 'test'+i);
                
                vdDocs.add(v);
        }
                
                Test.startTest();
                
                // run as different user
                System.runAs(u) {
                insert vdDocs;
                }
            
                Test.stopTest();
                
                //query for all documents created and assert that the account number was added
                for (VDDocs__c vd : [Select Account_Number__c from VDDocs__c where OwnerID = :u.Id])
                    system.assertEquals('001234', vd.Account_Number__c);   
    }
    static testMethod void testUpdate() {
            // create user to run the test as
        User u = [select id from user where name = 'Lindsay Kelly'];
        
        // create test account
        Account testAccount = new Account(Name='New Company', AccountNumber = '001235', Type = 'Client');
        insert testAccount;
        
        String accountID = testAccount.id; 
        
        // create a new document
        // test single insert
        VDDocs__c v = new VDDocs__c(
                Name = 'new test doc' ,
                OwnerID = u.id,
                Account__c = AccountID,
                Work_Order_ID__c = '77887788');
        
        insert v;
        
        Test.startTest();
                
        System.runAs(u) {       
            // create test account for update
                Account testAccount1 = new Account(Name='New Company', AccountNumber = '001236', Type = 'Client');
                insert testAccount1;
                String accountID1 = testAccount1.id; 
        
            VDDocs__c vd = [select Account__c from VDDocs__c where id = :v.ID];
            
            vd.Account__c = accountID1;
            
            update vd;
        }
        Test.stopTest();
        
        // query for updated doc
        list <VDDocs__c> updatedDocs = [SELECT Account__c, Work_Order_ID__c, Account_Number__c FROM VDDocs__c WHERE Account__c = :accountID];
                    
        for(VDDocs__c v1: updatedDocs){
            system.assertEquals(accountID, v1.Account__c);              
        
      }  
    }
}

Here's the trigger code:

trigger UpdateAccountNumber on VDDocs__c (before insert, before update) {
    if (Trigger.isInsert){
        // create a set of all the unique accountIds
        Set<id> accountIds= new Set<id>();
        for (VDDocs__c v : Trigger.new)
           accountIds.add(v.Account__c); 
       
       // create a map for a lookup / hash table for the account info
        Map<id, Account> accts = new Map<id, Account>([Select AccountNumber from Account Where Id in :accountIds]);  
         
         // iterate over the list of records being processed in the trigger and set the AccountNumber
        for (VDDocs__c a : Trigger.new)
                a.Account_Number__c = accts.get(a.Account__c).AccountNumber; 
    } else{
        if (Trigger.isUpdate){
        
            Set<id> accountIds= new Set<id>();
            for (VDDocs__c v : Trigger.new) {
                if (Trigger.oldMap.get(v.Id).Account__c != Trigger.newMap.get(v.Id).Account__c) {
                    accountIds.add(v.Account__c); 
                }
            }
            
            // create a map for a lookup / hash table for the account info
            Map<id, Account> accts = new Map<id, Account>([Select AccountNumber from Account Where Id in :accountIds]);

            for (VDDocs__c v : Trigger.new) {
                if (Trigger.oldMap.get(v.Id).Account__c != Trigger.newMap.get(v.Id).Account__c) {
                    v.Account_Number__c = accts.get(v.Account__c).AccountNumber;
                }
            }
        }                
    }
}

I don't really know what I'm doing yet and am planning on going through the lab exercises. Any advice and help is greatly appreciated.

Wendy