The audience with Benedict XVI is unusual for several reasons. The first is that Tuesday is usually a rest day for the Pope. In addition, papal audiences are usually reserved for heads of state, and not leaders of political parties.
Another reason is that the Communist Vietnamese government and the Holy See do not maintain full diplomatic relations. The two states have established a Joint Working Group to discuss the start of full relations, and the Vatican has a non residential representative.
One of the biggest sticking points in talks is the persecution of Catholics throughout the officially-atheist South East Asian country. The most recent Christian persecution list from Open Doors, place Vietnam at 21, describing the state of persecution as “severe.” The ecumenical group says Catholics and other Christians in the state are often harassed, and their worship increasingly restricted.