Monetary policy is not apolitical, as much as neoliberals will try to make it look like so. When a central bank increases interest rates, it's the poorer who can't buy houses, and can't get credit; it's the poorer who will receive fewer and lower quality public services, etc. There is no way to disconnect one thing from the other.

Furthermore, as others in the comment section have noted, VAT is an extremely regressive tax. The poor spend all their money. The rich spend close to none (and arguably will spend exactly 0 of their own UBI), and are the ones who benefit the most from consumption.

The poor will pay for their own UBI, plus for the UBI of the rich, plus for the products and services they have to consume which as noted will make the rich richer.

Your proposed scheme will expand inequality. Like I said, none of this is or can ever be apolitical.