arXiv:2610.23484v1 Announce Type: new Abstract: Uncertainty quantification for the Vlasov--Poisson (VP) systems entails repeated dynamic simulations, incurring immense computational costs. We develop a structure-preserving Semi-Lagrangian (SL) b-fidelity approximation of macroscopic observables. The MUSCL is used as a low-fidelity solver for pressureless Euler--Poisson system, allowing for a more cost-effective exploration of the stochastic parameter space. A conservative SL scheme combined with high-order discontinuous Galerkin is used as a high-fidelity solver for collisionless VP system, whose mass conservation, positivity preservation, and low numerical dissipation provide a more reliable high-fidelity snapshots for bi-fidelity reconstruction. We further derive a conditional error bound that separates projection, discretization, and model errors. Numerical experiments for both deterministic and uncertain VP problems, including multispecies and higher-dimensional settings, demonstrate that the proposed framework achieves accurate and robust uncertainty propagation.