LONDON/HANOI -- Farmers in major coffee exporter Vietnam are refusing to deliver coffee they have sold unless contracts are renegotiated following a surge in global prices to a 28-year high. Vietnam is the world's largest supplier of robusta coffee and the standoff is adding impetus to a rally in robusta prices with supplies in Europe becoming very tight. The poor crops drove prices much higher and farmers felt they could not afford to deliver at the agreed prices and have sought to renegotiate their contracts. Although much of that coffee has since been delivered, it has left less coffee available for sale this season, exacerbating the price rally. Mentality shiftOne Europe-based trader said he received just 20% of the Vietnamese coffee he was meant to get last November and December.