I've had some experience of this lately that I would like to share in case it helps anyone else in the same position.
For those who don't know, a vulnerable persons election, if granted, means that the trust is treated more favourable by the inland revenue than a discretionary trust without one.
The form to complete this is called a VPE1, and can only be completed online. There is no paper version. There is a fault in the form that means you have to input an address into the section concerning property assets held, even if there aren't any, otherwise the form won't scroll to the next section. HMRC technical support told me to complete this with my own postcode, reassuring me that if I wrote on the form when printed, and signed it, that the trust was cash only and thereabouts for putting the property details in, this would be sufficient.
In the section detailing who benefits from the trust I detailed that as a family trust, under the trustees discretion, I could gift £3000 to other beneficiaries although the trust was primarily for the benefit of my son.
Unfortunately it was this statement which meant HMRC declined my application for the election.
What followed was a rather fraut conversation culminating in my suggestion that as this benefit was a legislative one, HMRC appeared to be acting discriminatively.
After another "please hold the line" I was informed that yes, I could now have the vulnerable person election applied. I was told this legislation has been in effect since 2013. I was also told that this information will be cascaded to the team in future.
So if anyone has applied for a vulnerable persons election for their family trust and been declined for the same reason, I suggest you reapply! You should get a better result. And not pay such a high rate of tax on (any) interest you are lucky enough to earn......