The rents will need to go to servicing the loan (if you can find a lender), paying maintenance costs, paying the property manager and paying the pension provider. Unlikely to be anything left to invest in ETFs.
And don't forget it's not a sensible investment, unless part of a much larger overall portfolio.
And now I have a vague notion that there may now be a restriction on investing more than 50% of a pension in property (or is that just for exec pensions?)
Yes, at a maximum. Loan must be capital and interest and maximum term is the shorter of your retirement age or 15 years. I haven't looked at lending for one of these for several years. There used to be only one lender, Dilosk trading as ICS who lent to self-administered pensions and the rates were always a couple of percent higher than homeloan rates. Overall it's an awful lot easier, less risky and cheaper if you can buy without borrowing, even if that means holding on for a while until you have a bigger fund accumulated.
You'll need to leave a certain amount of money in cash in a bank account of the pension fund to cover expenses (liquidity), but if there's enough free cash then yes you can invest in Vanguard ETFs.
And now I have a vague notion that there may now be a restriction on investing more than 50% of a pension in property (or is that just for exec pensions?)
IORPS II did bring in a restriction on Occupational Pension Schemes (including Executive Pensions) investing no more than 50% of assets in unregulated investments (including property). This doesn't affect self-administered PRSAs, Personal Pensions, Buy-Out Bonds, ARFs etc.
I know a couple of people who borrowed through their pension fund and lost it all when property crashed.
The restrictions now make that a lot less likely. However, I really don't think that pensions should borrow at all other than maybe 10% or so to buy a particular property.
But even then, putting all your pension fund in just one asset? Do you really think that is a good idea? Maybe if you are 30 and will diversify your wealth over time into other assets.