Damn my crystal ball was working well last night!
OK, now we have the full set of information, we can start to work on the issue. F.75 is a common problem that plagues Vaillants, and can be caused by a whole host of issues, so the cure isn't immediately obvious and can get expensive. It's not all doom and gloom, however, as the initial attempts to remedy it are reasonably cheap...
I'll note before I say any more that you shouldn't technically be trying to DIY any of this, as you have to remove the front cover of the boiler which will break the room seal
First thing to check is that the expansion vessel is serviceable and correctly set. If it's burst, you need a new one, and probably a new pressure relief valve while you're at it
Next, the pressure sensors on these are rather sensitive to things like dirt, or it being a Thursday. There's a filter on the pipe that passes the sensor, and this should be removed and cleaned along with the sensor itself. A note should be taken of the quality of the system water; if it's brown, black, or has large particles in it, F75 will continue to happen until you get your entire heating system powerflushed. If the water and filter are clean but F75 continues, the pressure sensor should be replaced (I'm not aware of any means of testing it accurately to see if it's working correctly).
If replacing the pressure sensor doesn't work, things are going to start getting both more expensive, and more hit-and-miss. Your boiler has rubber hoses in the internal heating circuit, which are prone to degrading and clogging up (they can also burst, which is even more disastrous). If these are clogged, you'll get F75, and they'll need to be replaced. Vaillant will sell you an F75 kit for about £150, which replaces one of the hoses with the copper pipework your premium-priced boiler should have been made with in the first place, and moves the pressure sensor to a location where it's a bit less likely to get filled with dirt. I'm quite certain that this used to contain the copper pipework to replace both rubber hoses, but it seems that this is no longer the case. However, if your boiler engineer knows what he's doing he should be able to get both replacement pipework sections for you for the £150. It's worth checking that the auto bypass in the boiler is set correctly at the same time
If, having cleaned the system, corrected Vaillant's poor design and put proper pipework in with a pressure sensor in a sensible place, you discover that you STILL have F75 errors, then you probably have a duff pump. It might have had some muck in it, which has made it less efficient, especially if it was an old Wilo one. If you are unfortunate enough to have an old Wilo one, you will need to buy a replacement pump kit as the new pump is a Grundfos one and won't fit the hydroblock the Wilo is attached to, so you're suddenly replacing a good chunk of the water section of the boiler.
Finally, if you still get errors and haven't developed a drinking habit, you're looking at the PCB as being the problem. It is extremely rare for the PCB to throw up this issue, but not impossible; if it's not reading the message from the pressure sensor correctly, it's not going to let the boiler fire.
My advice? Once you've confirmed that the system water is clean, the expansion vessel is serviceable and correctly set, and the pressure sensor has been replaced, you might be as well off asking Vaillant for a quote for a fixed-price repair, as an independent could get expensive. They'll then keep throwing parts at it until it works again, although I must stress that they will only guarantee their repair if the system is clean, so if you have dirt in your system a powerflush and possibly a Magnaclean fitting will need to be carried out first
