The modulation of exchange interactions in layered magnets has fundamental research value and potential applications in spintronics. Based on first-principles calculations, the exchange interactions in the experimentally controversial room-temperature ferromagnetic 1T-VSe2 monolayer are systematically studied. It is found that three shells of nearest-neighbor Heisenberg exchange interactions and higher-order interactions are crucial for an accurate description of the magnetism and its thermal stability in the 1T-VSe2 monolayer. Based on our understanding of tuning the magnetic interactions and the magnetic ground state in the 1T-VSe2 monolayer via external factors, two modulation methods, involving adsorption of the alkali metal lithium and the electride Ca2N substrate, are proposed. In both Li-VSe2 and VSe2/Ca2N systems, the strongly frustrated Heisenberg exchange interaction competes with the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy, leading to complex magnetic ground states, such as antiferromagnetic spin spiral and periodic antiferromagnetic cycloidal states. Moreover, the higher-order exchange interactions play a crucial role in the stabilization of long-range double-row-wise antiferromagnetic states in Li-VSe2 and VSe2/Ca2N. These results highlight the effective manipulation of exchange interactions in two-dimensional magnets.