visa on arrival with emergency passport

Hi there,
I'd like to apply for visas on arrival with myvietnamvisa.  I'm aussie and by boyfriend is from the UK, only problem is he's waiting for his renewed passport to arrive and we leave on the 24th May. What  is the quickest time you're able to send out an approval letter? And are you able to offer visa on arrival for emergency passports?
Thanks in advance for your help.
Regards,
Rosie Blood