The reason they were greeted by "semi-official" personnel is that Vietnam and Taiwan do not have formal diplomatic relations. Since the Vietnam War, Vietnam has remained a socialist country, so politically it has to pay a lot of attention to mainland China.
說是「半官方」人士的禮遇，主因是越南和台灣沒有邦交，自從越戰之後，越南目前仍是社會主義國家，政治上相當程度需顧及中國大陸。