Good question. I speak with a northern accent and I would use 'lien lac' as well. I am not sure as my Vietnamese is only so-so, but I would say there is a difference. I understand 'lien lac' as 'to contact' or 'to communicate with someone (by phone etc.)' but 'lien he' more like 'having a relation/something to do with someone'. But I might be totally wrong.
My understanding was that 'Làm sao' would be a more appropriate question when asking a question like this, while 'như thế nào' is more about asking for descriptions of things. Perhaps I was misguided. Can anyone explain how these two question words differ? As they both translate as 'How'.