If I don't receive an invoice for service from a vendor in a timely matter do I still have to pay it?

Unless your contract with a vendor specifically states that the vendor is required to send an invoice within a specific time period, the vendor may still be able to send an invoice despite the delay. 

There are two potential defenses to a late invoice. 

First, the deadline for the vendor to sue on the invoice would be two years after the payment became due if there was no signed contract, or four years after the payment became due if there was a signed contract.  This raises the question: "When does a charge/invoice become due if no invoice is ever sent?"  Arguments exist that the clock starts as soon as the materials/goods were provided. 

Second, the legal doctrine of "laches" might apply here.  "Laches" is a broad concept that applies when one party unreasonably delays the enforcement of a legal right, and the other party suffers as a result.  Your letters to the vendor might help you if they say, "We will not be able to pay for these charges unless you provide an invoice by (DATE)" and you provide a reason why, such as your accounting on the job needs to be closed.  This may or may not be successful in establishing the defense of laches, but it would certainly help.

Unfortunately, the vendor will still have an argument that the amounts are owed so long as the deadline to sue is met.  But you will have more arguments in defense of the claim if you can demonstrate that you were adversely affected by the delay AND that you provided notice to the vendor of such adverse effect.

Hope that helps...

David Barnier, Barker Olmsted & Barnier APLC