"My Vietnamese colleagues explained it this way, when you burn these offerings of money, motorcycles, clothes, etc etc, the smoke wafts its way to the heavens where the relatives all live now and they may need these things in the after life. At the time of Tet, this smoke goes up to the Kitchen Gods where you have spent days making your home more acceptable to them. It is a Chinese custom..."
From the www:
You may also see DVD players, TVs, jewellery, food and clothes being burnt...
They also burn incense when giving these offerings.
They also do this at other times of the year, an anniversary of someones death, and on Tomb Sweeping day."