Do not wrap a gift in black paper because this color is unlucky and associated with funerals Vietnam. Gifts that symbolize cutting such as scissors, knives and other sharp objects should be avoided because they mean the cutting of the relationship.
Vietnamese may or may not open these gifts when they are received; leave the option to them.
You will also receive gifts and should defer to your host as to whether you should open it when received or not. Regardless of when it is opened or what it is, profuse thanks are always appropriate.