The ETFs should be OK as well as they should be held in a separate account with a depository holder the same as the cash. The investor compensation scheme is meant to cover cases where the broker did not invest your money in a fund but used it to cover his expenses. Not supposed to happen as the Financial Regulator is supposed to check that all broker hold clients monies and assets separately but history shows that it can.
I don't remember ever having heard of a reputable fund going bust although some money market funds did when they promised returns that could not be met when interest rates went to zero. If id did happen, then you would be in the same boat as a shareholder whose company goes bankrupt - last in line for any assets. It is hard to imagine a scenario where a Vanguard fund goes bust.