Hi Andrew,
I encountered the issue of slow convergence using VUMPS in exactly solvable anisotropic Kitaev model. Having already invested significant time in writing my own iDMRG code, I ultimately focused on getting it to work instead of pursuing the VUMPS method further. In the case of exactly solvable Kitaev model, I encountered convergence issues with both VUMPS and 2-site iDMRG. I resolved the issue by adding subspace expansion to the 2-site iDMRG (see DMRG3S), so I guess the convergence issue encountered in VUMPS can also be resolved by implementing subspace expansion of some sort. Yet another way is to add “noise” by perturbing the MPS as delineated here density matrix perturbation. See Matt’s comment in https://itensor.discourse.group/t/difficulty-with-simulating-a-specific-model-with-vumps-that-works-perfectly-fine-with-regular-dmrg/2027/4.
Best,
Xu