Coupling the vehicle-to-grid (V2G) with integrated energy systems (IES) offers an emerging solution for decarbonisation of both energy and transport sectors. To evaluate the feasibility of coupling V2G with IES as a flexible storage, we propose an optimisation-based system planning framework embedding V2G into IES. Within this framework, stochastic features of electric vehicles (EV) fleets are simulated. The impacts of V2G on IES design are captured by assessing both economic and environmental benefits via multi-objective optimisations utilising an improved NSGA-II algorithm. Six case studies considering three cities with different climate conditions and two functional areas of residential and commercial are performed. The results manifest that Beijing-commercial case could achieve the largest mutual benefits. The EV fleets’ charging behaviour follows the time-of-use energy tariff in transition seasons while not during winter. Sensitivity analysis indicates the electricity and gas prices have significant impact on the system design. The benefits induced by growing EV penetration would gradually decrease and stabilise when the EV number reach 300, the growth of economic and environmental benefits stabilized at 1.3% and 1.8%, respectively. Overall, this study quantifies the benefits of enabling V2G in IES, and generates valuable insights for IES planners, V2G service providers, and relevant policymakers. Display omitted •Co-optimisation of EV vehicle-to-grid (V2G) with integrated energy systems (IES).•Simulate stochastic features of EVs' charging patterns and initial battery states.•Capture V2G-IES design's trade-off by an efficient DM-NSGA-II algorithm.•Analyse parametric sensitivity of electricity price, gas price, and EV penetration.•V2G-IES achieves the largest mutual benefits in a Beijing-commercial demonstration.