I have a letter of invitation from the Consul of the Netherlands in Hanoi as I will be staying at a diplomatic residence. No where on your online application for Vietnam visa did you ask for this. Please advise. 

Carol Ann Rein,

Dear Carol Ann Rein,

In case you apply for Vietnam visa on arrival online with us, you do not need to provide this information in the online Vietnam visa application form, only filled the form with required information, you will get your visa approval letter (or Vietnam visa invitation letter) via email after 1 or 2 working days.

You can see full details about this kind of visa for Vietnam here: https://www.vietnam-visa.com/how-vietnam-visa-on-arrival-works/ .

Kind regards,