Vietnam is not currently VTL but I would guess would have a high chance of joining if Vietnam opens up to Singapore travelers with no or little quarantine. Typically, short term visitors from non VTL/China/Taiwan are not allowed entry but I believe there are some waivers for critical medical tourism if there is an existing patient relationship subject to the typical quarantine requirements for those travelers (currently 7 days self-isolation at home/hotel). Unless this is really urgent and has to be done in Singapore it is probably simplest to wait for a Vietnam VTL or go elsewhere.