Can I apply for a tourist visa for China from Vietnam if I'm a UK National?

My boyfriend and will be in Vietnam in August and want to travel to China by train for sightseeing and to visit a friend in Hong Kong. He is a Chinese national and so can provide an invitation letter. However, is it possible for UK Nationals to get tourist visas in another country to their own? I can't find any information to say whether we can get one from Vietnam. We don't have time to get one in the UK as we're leaving the country in 2 weeks to go to India. Thanks