The traveller is left with an uncomfortable choice: to return to an airless hotel room with a rickety fan, or to wander the streets on a seemingly futile quest for shade and resuscitation. T-shirts become sodden with sweat, legs grow weary.
This is when you should make for the Tam Tam Cafe. Here the "whoop whoop whoop" of the first effective fan I have encountered in two weeks of travelling dries the sweat off my back. My freshly squeezed lime juice and soda brims with ice. The peanuts are dry roasted. The sofa is soft and deep. I am never ever, ever, going to leave this place.
"Once in a while, everybody needs to feel at home when they're travelling. If they say they don't then they are lying," says Christophe Brasseur, the French proprietor of this sanctuary.
Three years ago, this lanky 32- year-old sold his windsurf business in Normandy, bought a motorbike and rode to Vietnam. Within a year, he had arrived at the ancient town of Hoi An. Gently sprawling along the banks of the Thu Bon river, just two miles from the South China Sea, Hoi An has relinquished its position as a major seaport to the more savvy South Vietnamese.
Today it beguiles tourists - just as it did 20 years ago when both the Americans and North Vietnamese, obligingly, left it well alone. Its narrow streets and crumbling verandas give the town a French charm. There is a local delicacy, a meaty noodle called cao lau, which is famous throughout Vietnam. But the main attraction is its sleepiness: Hoi An is a backwater, ignoring the clamour that has gripped the rest of the country, feverishly trying to catch up with its Asian neighbours.
Brasseur discovered another key ingredient. Hoi An is equidistant to Ho Chi Minh in the south and Hanoi to the north. Whichever city travellers fly into, they almost always gravitate towards Hoi An. Brasseur reckons that most people tend to arrive here after 10 days in the country. "Sure, we all want to eat noodles and drink warm beer. But, come on! After a while, you have to have a change. You want to get away from the noise and you want to eat food you understand."
He started trying to persuade his girlfriend, Natalie, to stay and help just as Vietnam was waking up from its Communist torpor. Initially, the small-town politics were baffling. His reception by local officials was polite but unproductive. When he pushed, he met with prevarication and cups of green tea.
What he needed, he discovered, was a local sponsor. He was introduced to a powerful Chinese family that had lived in Hoi An for generations. The patriarch became his translator and secured a building in the centre of town. The three-storey house was in dire need of renovation.
"The local builders were superb, very skilled. But they have a different mentality. Like, I wanted a lavatory that was comfortable. That people could sit on. How many times can you do that in Vietnam, eh?"
"But they make this tiny 'ole. I tell them, `This is ridiculous. One hundred and fifty people every night using this tiny 'ole? Crazy!'.
"Three times we try before they understand. Then they make me a wonderful 'ole."
It is a wonderful 'ole. There is a seat. It is clean and white. There is soft loo paper. There are individual towels at the basin.
As I drift back to the bar after my heavenly lavatorial experience, a young Mancunian who I had met a few days earlier hails me.
"Alright, mate? What d'ya reckon to this place then, eh?"
"I think it's brilliant."
"Do ya? No, it's dead here, mate."
He has missed the point. Tam Tam Cafe is not here to set the world alight. This is the place we dream up on the back of an envelope on a wet Wednesday morning on the Northern line, when the strain of metropolitan life makes us yearn for an alternative. The floors are dark wood, the bar is mahogany. Rattan tables hold copies of ParisMatch and chess sets. There's a billiard table. At the bar, Glenfiddich and Wild Turkey compete for space with Scrabble sets.
At the front of this vast open space is the restaurant. The Frenchman avoided any local antagonism by opting for a western menu. There is steak from Australia and claret from Europe. Homemade lasagne, avocado salads and lemon tart with thick cream. This luxury is not cheap. Dinner here will set you back $20-$30, a small fortune for Third World travellers. Brasseur is unrepentant. He does, after all, have to import everything.
There is genuine pleasure sinking one's teeth into a steak after two weeks of noodles, but it is at dinner that we discover the Tam Tam's weak point. My companion's pasta is stodgy and the puddings are bland. Brasseur says he has spent the last two weeks in the kitchen of a friend's restaurant in Hanoi, picking up tips for his local staff.
"We are not there yet, but the cooks are working hard and get better every day, and my supply lines are running more smoothly."
It is easy to forget that he is working more than 400 miles from a major city, in a country that admitted outsiders just six years ago. He and Natalie are the only westerners in town.
"It is lonely sometimes. But I love the Vietnamese. It is why I am here. Not only to make money."
Despite his obvious love for the country and the success of the cafe - Brasseur employs 18 local staff and his bar is rarely less than full - the Frenchman's future is uncertain. Nothing is written on paper to say either that he lives in Hoi An or runs a restaurant. He's been refused permission to buy a house and he and Natalie still live in the old state- run hotel on the edge of town.
"I know that they could stop this at any time. Just walk up the stairs and say `Goodbye'. That's fine as long as you know that your business hangs on a thread. You make it last as long as you can and, when you leave, you have no regrets. This is Vietnam. Not Paris."
Application forms for tourist visas are available in person or by post from the Vietnamese Embassy, 12 Victoria Road, London W8 5RD (0171-937 1912). If applying by post, include a stamped, self-addressed envelope.
The office is open from 9am-12 noon and 2-6pm, Monday to Friday. A one- month visa costs pounds 40, payable in cash or by cheque, and takes at least a week to obtain. You need two photographs.
Fares to Ho Chi Minh are very competitive at present. Flightbookers (0171- 757 3000) quotes a price of pounds 460 return from Heathrow on Thai Airways International via Bangkok in November, while Quest Worldwide (0181-547 3322) is offering a fare of pounds 582 on Air France from various UK airports via Paris.
Other carriers to Vietnam include Lauda Air (via Vienna), and Cathay Pacific (via Hong Kong).
Cheaper flights may be available on Aeroflot (via Moscow) through discount agents.
No vaccinations are compulsory, but precautions are recommended against tetanus, polio, cholera, typhoid, hepatitis and malaria. If you are staying in cheap hotels, a mosquito net will considerably enhance your prospects of sleep and reduce the risk of mosquito-borne diseases.