Single payer was supposed to be cheaper and it turned out to be more expensive. How does "all-payer" save any money? It looks to have all the disadvantages of single-payer with none of the advantages.
Single-payer isn't "more expensive," it requires substantially higher taxes because virtually all health services get paid for via tax revenue (not money collected via private insurance premiums). Raising taxes that much, even if the increase is offset by the complete lack of health insurance premiums, is a political non-starter.
Their all-payer system is based on what already exists. If you're in private coverage, you're still in private coverage. If you're on Medicaid or Medicare, you still are. The difference is that all of those payers start to pay health care providers the same way--a way that allows the state's hospitals and doctors to function more efficiently and deliver better care in a more sustainable way.
One of the advantages of a single-payer system is that health care providers get a consistent signal from the health insurance industry--because there's only one health insurer. In Vermont's all-payer system, providers will still get a consistent signal because all of the payers will be sending the same one.