We are at Madam Phuong Banh Mi restaurant in Hoi An, a small town in Quang Nam Province, on the country's central coast. The old quarter of this former Chinese and Japanese trading port is heritage-listed and in excellent condition, thanks to cooperation on both sides during the American War.

Today, squadrons of cyclo drivers pedal selfie-stick wielding tourists down its enchanting, narrow streets. An array of tailors and dressmakers tout the same holiday makers pot luck replicas of famous fashion labels and strings of colourful bamboo lanterns hang everywhere (also for sale, if you're interested).

But Hoi An is increasingly drawing visitors for its food. Although a long way from the rich Mekong River food bowl in the south, this relatively sleepy town has developed a reputation for local delicacies, such as translucent 'white rose' dumplings, pork cao lau noodles and com ga (chicken rice); decent local and tourist-friendly restaurants and cooking courses abound here, too.

An accompanying surge in food blogging, travel apps and social media now means visitors are eager to ensure they can boast about having tried the 'best' place for this dish or other. Consequently, trails to certain street food vendors increase, driving more tourist traffic, leading to more Instagram posts and so it goes.

Which brings us back to Madam Phuong's. If you ask a taxi driver where the best banh mi in Hoi An is they will mention this spot at 2B Phan Chu Trinh Street and one other: Madam Khanh "The Banh My Queen", an octogenarian who owns a popular stall at 115 Tran Cao Van Street. Banh mi eaters are divided as to which is better, although on the internet, Madam Phuong generally wins out, thanks to a mention on Anthony Bourdain's No Reservations television series (he described her banh mi with the lot as a "symphony in a sandwich").

When I finally make my way to the front of the queue I am confronted by five ladies working in seemingly fast motion behind a glass counter. They diligently avoid eye contact as they put together orders in a blur of chopsticks and ingredients: pate, mayonnaise, coriander, pork cold cuts, roasted pork, pickled daikon and carrot, chilli, sauces and, of course, stacks of golden bread rolls. As tourists pose for photos with their famous sandwiches, I smuggle mine outside, away from the gastro-paparazzi. Biting through its brittle outer crust it shatters, giving way to a fluffy white, almost otherworldly interior. Flavours zip and sing on my palette - savoury pork, fresh coriander, crunchy cucumber - and then the chilli hits, like an old, welcome sparring partner. It's good. It's very good. But is it the best?

Literally translated, banh mi means 'bread' or 'wheat cake'. What most Westerners recognise as a Vietnamese pork sandwich here is called banh mi thit ngoui ('bread, meat and cold cuts') or banh mi dac biet ('the special'). Its evolution is a fascinating tale, spanning Vietnam's French colonial past and three bloody conflicts.

In a condensed version of a 10,000-word treatise on the subject, writer Simon Stanley traced the modern origins of banh mi to an address in Ho Chi Minh City's District 3. Run by a Hanoi family who fled to the southern city (then known as Saigon) when the country divided in 1954, Hoa Ma is believed to be the first banh mi shop to reduce the size of their rolls to the now familiar 20 centimetres; they also substituted vegetables for expensive meats, to make the sandwiches more affordable for Saigon's working men and women. "It's a leap to say that they invented the banh mi," says Stanley, 33, who was born in London but now lives in Saigon. "But I think as far as what you see and eat today, it's very likely that they started putting that together first."

Back in Hoi An, I cycle through the backstreets to sample Madam Khanh's famous banh mi. It is a completely different beast: rich and hearty, it oozes a kind of gravy sauce that runs down my arm as I eat. It's delicious, but as I wipe the grease from my chin I wonder, 'Is it better than Madam Phuong's?'

In truth, the perfect banh mi is a kind of mirage. After three months of travelling the country, the best sandwich I taste is from a roadside vendor in Danang. But I won't tell you the address (it was near the beach). Instead, I implore you to go in search of your own, personal banh mi. It's a lot of fun, and tastier, too.