At this market, said Tuong, young tribespeople gathered from all the surrounding hill settlements - H'mong in their vivid indigo tunics, Giay in exquisitely embroidered aprons, and Zao in their scarlet turbans, eyebrows shaved and ears heavy with silver hoops. The weather is so critical as there is little around Sa Pa that can be called a road, so the market- goers have to scramble through rice terraces, along mud tracks and across bridgeless rivers to reach the town, which becomes completely cut off during the heaviest rains.
But they persevere, grinned Tuong, because after spending the daylight hours hawking around their heavy jumbo coops of fat, complaining chickens, their baskets of onions, eggs, herbs and pineapples, and pressing into uncomplaining hands their tiny boxes of illegal opium, they can finally relax, crack open the rice wine and prepare for some real fun - because the young people are encouraged to pair off publicly for a night of unspecified and anonymous romantic activity in the doorways and derelict buildings of the town.
According to Tuong, a stylish Hanoian with two gold teeth and four very long, curly and clearly prized hairs growing out of his neck moles (a fashion much admired among the male Vietnamese population), the women who are looking for a night of pleasure identify themselves to potential lovers by wearing a single glove and carrying an umbrella.
At first we put this information down to a combination of tale-telling for tourists, and snobbery. The dozens of ethnic minorities who have accounted for a sizeable chunk of the country's population for centuries are nevertheless still regarded as second-class citizens by the Vietnamese, and the notion of unprincipled tribes indulging in some sort of primitive hillside orgy would be a typical reinforcement of their caste prejudices. Yet after independent inquiries, Tuong's description was corroborated by so many other sources that the quest became irresistible and we packed our warm clothes for a detour north.
Few visitors venture to the furthest reaches of Vietnam. They tend to stick to a tried-and-tested track, flying into the southern capital, Ho Chi Minh City, and travelling up as far as Hanoi, drinking in a long sweep of the bamboo and buffalo coastline either on the Reunification Express or on one of the ancient minibuses that rattle up and down the infamous Highway One. So we had no difficulty bagging the best bunks on the overnight sleeper from Hanoi to the Chinese border town of Lao Cai, 300km north at the end of the line.
As foreigners, our hard berths cost us 110,000 dong (about pounds 10), four times more than the locals' rate. Our gentle carriage companions stared at the printed dockets for hours, round-eyed with disbelief that we had paid so much for the journey.
Lao Cai was sodden, and the two-hour bus climb to Sa Pa truly terrifying as we struggled and skidded along the narrow mountain passes. It was beautiful, certainly - valley after valley of swirling rice terraces as far as the eye could see. But it wasn't just the conductor's piglets which squealed as we rocked against the wind and ploughed into the streams.
Only once did the weight of the passengers defeat the vehicle: in the middle of a fast-flowing brook, we all had to pile out and wait on the far bank for the men to push her across. Yet the bus made it to Sa Pa, which was quite lovely even when bathed in an all-embracing fog, and we soon settled into a guest house to read, keep warm, drink the local rice wine and await the following night.
We pieced together more information about the Loving Market the following morning as, in spite of rapidly worsening conditions, commercial activity got into full swing. The tribeswomen, we were told, have to prove their fertility before they can be considered marriageable, and so the Loving Market provided them with an opportunity to do so. Once pregnant they could become betrothed, and their marriage partner would happily accept the first child as his own.
Whatever the truth of this, the issue was very much on the minds of the slender H'mong women who, since the opening up of the area to Westerners in 1993, had made it their business to absorb a few words of English. The tiny tribeswomen - we didn't see one over 5ft - crowded around our waists as we braved the rain and ventured through the market. Twittering like excitable sparrows, they waved embroidered goods under our noses while calling out a single mantra. "You married? You children? How many?" Stupidly, we said that we had none, and their faces suddenly softened with compassion as they stroked our arms, speechless with pity.
By nightfall most of the minorities seemed to have left Sa Pa - the fog had thickened further, terrible rains were expected and they had not wanted to risk being stranded in town, possibly for days. Some locals said the Loving Market had been cancelled altogether, others that it had been postponed for a night, or that a scaled-down one would be held, but on a drier site on the other side of town.
Visibility worsened and before giving up the search we wandered around the few damp streets, watching the bars and cafes slowly disgorge their customers as Sa Pa settled down for the night. On the way back to the guest house I heard a shy giggle from a doorway, and looking up I could just make out two tiny blue figures, hand in hand, emerging from the fog. One playfully prodded the other with the point of an umbrella, before they both darted across the courtyard and back into the shadows.
Voyaging to Vietnam
Getting There: Ruth Cowen paid pounds 582 on a Singapore Airlines flight into Hanoi and out from Ho Chi Minh City (Saigon). Cheaper discount flights to either city are available on Aeroflot (via Moscow), Thai (via Bangkok) and Cathay Pacific (via Hong Kong).
Foreigners' tickets for a "hard berth" (basic, but comfortable) on the overnight sleeper from Hanoi to Lao Cai cost about 110,000 dong (pounds 10). Weather permitting, the Sa Pa bus departs from Lao Cai railway station each morning, for 20,000 dong (pounds 1.80).
Organised Tours: Regent Holidays (0117-921 1711) offers custom-made tours which can include China, Cambodia and Laos. Oriental Magic (01253 791100) runs add-on packages from Bangkok and Hong Kong to Vietnam; they take care of visa formalities.
Red Tape: Get application forms for tourist visas in person or by post from the Vietnamese Embassy at 12-14 Victoria Road, London W8 (0171-937 1912). It costs pounds 40, and takes at least a week. If you intend to travel between Vietnam and China, you also need a week to procure a Chinese visa (pounds 25), most easily through China Travel Service, 7 Upper St Martin's Lane, London WC2H 9DL (0171-836 3688). This agency charges pounds 15 for processing by post.
Health: No vaccinations are compulsory, but precautions are advised against tetanus, polio, cholera, typhoid, hepatitis and malaria. If you are staying in cheap hotels, a mosquito net is recommended.
Guidebooks: The Rough Guide to Vietnam (pounds 9.99, published last November), Lonely Planet's Vietnam: A Travel Survival Kit (pounds 9.95, published 1995), and their Vietnam Travel Atlas (pounds 5.95). Both Lonely Planet and Rough Guides sell a Vietnamese Phrasebook (pounds 3.50).