a regressive tax used to fund a progressive policy can be either net regressive or net progressive. if rich people buy enough stuff that they pay more VAT than they receive in UBI, the whole system is still progressive.
For a good example of this, see Social Security (OASDI) in the US. The funding method is very regressive, taking a flat percentage of income only under a threshold (12.4% of income under $137,700 with half paid by employer and half paid by employee). People who make more than that pay less tax.
But then we use the tax to directly fund one of the most progressive programs in the US, which directly pays people who need it most.
I think most people would think of Social Security as a net progressive policy, even though it has a very regressive funding model.
And really, a VAT might be slightly regressive in practice because poor people tend to spend a larger fraction of their income but it's still pretty close to being a flat tax. If you were to try to fund a UBI with a head tax that would indeed be pointless. As long as rich people tend to spend large amount of money, in absolute terms, than poor people a VAT+UBI combo is going to be progressive.
But then we use the tax to directly fund one of the most progressive programs in the US, which directly pays people who need it most.
I think most people would think of Social Security as a net progressive policy, even though it has a very regressive funding model.