Can I mix lithium and VRLA on the same bus during a staged replacement?

Generally no. Their float voltages and charge acceptance characteristics are incompatible, so a single charge regime under-charges the lead-acid or over-holds the lithium at full state of charge. For a staged programme, either separate the buses with independent charge control, or accept that one chemistry is operating outside its optimum and keep the transition period short. Plan the replacement bank-by-bank, not battery-by-battery.