just confirmed Id get yourself to HCM and book a hotel there so u can send them the booking. Then u can claim u arrived to Hanoi but travelled to HCM as thats where u would be when application was processing. Whether they accept this or say u were in Hanoi when applied so application is invalid, who knows! U can also email Hanoi to ask I guess but then u are letting them know the issue so up to u!
surely someone can arrive to HCM and show that stamp if applying in Hanoi but then travel to Hanoi and maybe show hotel booking in Hanoi? Or vice versa obv!
Check on the site to see if HCM accepts Hanoi applications. But also it should be fine as you could claim that you arrived to HCM but then travelled to Hanoi! Or just take a trip to HCM anyway so you are in fact there! I guess its all about the arrival stamp though!