It was explained to me that your money is in whatever Vanguard has invested it in. It would take some time to unravel but unless the world was ending the diversity of investments they make means you’d get your money back, or more likely some other platform would take them over (probably with a big fee).
However I did have money with woodford and when winding that up they sold off the investments too cheap to their mates to profit on leaving investors well out of pocket.
If you want something else then you need to look at the spread of investments your vanguard product is in and choose something similar. Or build your own diverse portfolio based on your personal risk and interest. But you would need to open an account with another platform and bear in mind ISA rules, and it will be hard to beat the fees and ease which vanguard offer.