invoices.csv

Vendor,Invoice Date,Invoice Amount,Invoice Status,Invoice Number
Acc-0,1/07/2014,5500,New,INV-0001
Acc-0,1/07/2014,18400,New,INV-0002
Acc-1,1/07/2014,4500,New,INV-0003
Acc-2,1/07/2014,3750,New,INV-0004
Acc-3,15/06/2014,6800,Paid,INV-0005
Acc-4,15/06/2014,14280,Paid,INV-0006
Acc-5,15/06/2014,9999,Paid,INV-0007
Acc-6,15/06/2014,3570,Canceled,INV-0008
Acc-6,1/07/2014,8400,New,INV-0009
Acc-7,1/07/2014,2700,New,INV-0010
Acc-7,1/07/2014,6995,New,INV-0011
Acc-8,15/06/2014,14300,Paid,INV-0012
Acc-9,15/06/2014,2850,New,INV-0013
Acc-10,15/06/2014,4600,New,INV-0014
Acc-10,15/06/2014,3700,Paid,INV-0015
Acc-11,15/06/2014,6400,New,INV-0016
Acc-11,15/06/2014,10340,Paid,INV-0017
Acc-11,15/06/2014,1300,New,INV-0018



List<Account> accounts = [select id, vendor_number__c from account];
for(Integer i=0 ; i<accounts.size() ; i++) {
   Account a = accounts.get(i);
   a.vendor_number__c = 'Acc-'+i;
   System.debug(a.vendor_number__c);
}

update accounts;