The mother of two, who had been waiting for a kidney transplant for five years, struck up a conversation with Veronica Buttigieg, a friend of a friend she had never spoken to before.
The talk turned to health and Mrs Viravongâ€™s disappointment that her brother had decided against donating one of his kidneys to her.
But Mrs Buttigieg, 53, did more than just politely sympathise. Over the clink of glasses and chatter of the other guests, she made an amazing offer.
â€˜Why not have one of my kidneys?,â€™ she asked.
Two months later, the operation took place and Mrs Viravong, 45, is well on the way to recovery.
Mrs Buttigieg, who lives with her husband Mario, 54, a painter and decorator, said she would happily donate again if she could.
â€˜I barely knew Manoly but after talking to her and hearing of how desperate she was, I felt it was the right thing to do,â€™ she said.
Mrs Viravong, who has suffered from kidney problems for 18 years, is still astonished at her good fortune.
â€˜I did not know Veronica very well at all. We have a mutual friend and I had seen her at social gatherings a handful of times but we had never really talked,â€™ she recalled.
â€˜Shortly after arriving at the party, which was a friendâ€™s 40th wedding anniversary, she offered me her kidney. Just in a really casual way.
â€˜I thought that she was joking, so I just said thank you. But then I realised that she was serious.
â€˜I asked her if she was really sure, and she said she was positive. Two days later she had gone to the hospital to be tested to see if we were compatible.
â€˜When I found out we were a match I was stunned. I just couldnâ€™t believe it.â€™
The women both live in Leigh-on-Sea, Essex, and the operations took place in July, on the first day of the school summer holidays, so Mrs Buttigieg, who teaches sick children at Basildon Hospital, could be back at work in time for classes to resume.
Her husband and sons Richard, 20, and Robert, 22, supported her decision, although they became increasingly anxious as the operation neared.
â€˜Mario is used to me,â€™ Mrs Buttigieg said.
â€˜When I offered Manoly my kidney, she turned to him and asked if it was all right. He simply said â€œif that is what she would like to doâ€.â€™
As the operation loomed, the family became more concerned, aware that any type of surgery involving a general anaesthetic always carries a risk.
â€˜We all knew there was a chance I could die,â€™ Mrs Buttigieg said matter-of-factly.
â€˜But I wasnâ€™t worried at all. I am a Christian and I felt that God said it was right. I had a real peace about it because I knew it was what God wanted me to do. I felt completely calm.â€™
Since the operation in July, the women have become the closest of friends.
Mrs Viravong, who is originally from Laos, is well enough to consider reopening the Thai restaurant she used to run with husband Sene, but closed due to her ill health.
She has two teenage children, Viumee, 18, and Sam, 16, and is relishing every moment with her family.Â â€˜It has been wonderful. I have got my life back,â€™ she said.
â€˜For two years I had endured eight hours of dialysis a night and my kidneys were deteriorating. My life expectancy was very short â€“ it would have killed me ultimately.
â€˜I had been feeling very low because my brother had changed his mind about giving me his kidney, yet here was a virtual stranger offering to do an amazing, wonderful, selfless thing for me.
â€˜I canâ€™t really thank her enough.Â We are very close now, like sisters. Iâ€™m really grateful to her and I count my blessings every day.â€™
As they waited at the Royal London Hospital before surgery, MrsÂ Viravong kept asking her new friend if she was definitely sure.
â€˜She said she was positive but she just wanted them to get on with it.
â€˜After that it was a few days before we could talk to each other again.
â€˜When I saw her, I just cried and cried. We held each other and I told her how grateful I was.â€™
By the time of the transplant, Mrs Viravongâ€™s kidneys were functioning at just 10 per cent of their capacity.
They remain inside her body, along with the transplanted organ.
Mrs Buttigieg is back at work, going to the gym and coping perfectly well with one kidney.
Although her safety net is gone if ever she develops kidney problems of her own, she is adamant that she has done the right thing.
She said: â€˜I have no regrets and I would do it again tomorrow.â€™