1. It remains to be seen whether John will come.
2. There has always been some controversy about her eligibility.
If Vietnamese is a null-subject language--and the typical absence of proforms in embedded clauses suggests this is the case--then it should not have expletives either. However, Phan Thi Huyen Trang (personal communication) draws attention to cases such as those in (3) and (4), in which, she suggests, the proform nó is non-referential, like English expletives:
3. Uống cho nó đã.
drink for it fill/quench
'Drink your fill (to quench your thirst)'
4. Tắm cái cho nó mát.
bath prt for it cool
'Take a bath to cool down'