If I upgrade and the line keeps dropping out - as it does now, but worse because VDSL is more sensitive - and they send another BT engineer round, and he says "no fault" - then I'm left with a faulty line, a long contract, and nobody will accept that the fault even exists.
Are you going to pay for my contract if that happens? Of course not. The ADSL can barely keep synced, VDSL on this line will be a joke, and reading reports online of people's experience when VDSL upgrades don't work, coupled with my own experience of OR, leaves me *certain* that I will not have a good time.
You'll have to forgive me for knowing a bit more about the quality of my line than you do. Given that you can give me no guarantees that the service will work, and given that it doesn't I will have no recourse to get it fixed - like I have no recourse to get it fixed at the moment - then I don't see the value in making things worse for myself.
Long story short - OR won't fix the line, you don't put VDSL on a line that can barely handle ADSL.