With a Vietnamese passport, you need to apply for both a Hong Kong visa and a China visa separately. If you're going for a short visit as a tourist, I don't think you necessarily need a letter from a sponsor in Hong Kong as long as you hold an ongoing air ticket to say Singapore. As many people travel between Hong Kong and China over land, an ongoing ticket out of either Hong Kong or any major Chinese aiport should be sufficient. The fact that you are a permanent US resident will help a great deal. See the following information from HK Immigration: