The geometrical structures and electronic properties of the defective silicene with monovacancy (MV), placed on two different substrates, h-BN sheet and Ag (111) surface, have been investigated by using the first-principles method. It is found that due to the weak van der Waals interaction from h-BN sheet, there exist still two kinds of MVs, i.e., the self-healing MV-1 and the MV-2 with a closed five- and nine-membered (5-9) pair of rings, in the Si|h-BN composite with the former as the ground state MV, which is the same as the case of the free-standing silicene with the MVs. The electronic bands around the Fermi level of the Si|h-BN composite with the Si MVs are all induced by the Si atoms, leading to a metallic composite. But the Ag(111) substrate has the stronger coupling with the Si adlayer, leading to existence of three kinds of inequivalent Si atoms, greatly changing the MV structures and stability of the defective silicene adlayer, as well as its corresponding electronic structures. For example, the symmetric-like MV are found to be always the stable MV structures in the Si|Ag(111) composite, no matter which type of inequivalent Si atoms is removed from it. And the Ag substrate has a dominant contribution to the electronic bands of the defective Si|Ag(111) composite with the Si MV. Even so, some characteristic bands induced by the Si MV defects could still be found.