If I apply for multiple entry visa and I have stayed through the allowed 30 days, can I leave the country and return within 24 hours and have another 30 days to stay?
You totally can do that.
Please follow the link to apply visa online with emergency case to have visa within 1 working day https://www.vietnam-evisa.org/apply-visa.html
Apply visa online, you will get visa stamp on the passport upon arrival, all you need to do is fill your information in the form and make payment online.
After that, we will send you the approval letter via email, please print it out to board the plane and use it to get visa stamp on the passport on arrival.
For any further details, please contact us at firstname.lastname@example.org call us at (+84) 909 343 525or +84.327.117.119