I can't find a picture of the connections, it does seem that it does have the option of e-bus control, but nothing about opentherm so likely you will need the controls supplied by Vaillant to use the e-bus, Nest can only switch it on/off, I could also find no pictures of internal fuses, often one has to take the 230 volt supply from inside the boiler or the thermostat can by-pass the fuse, but I could not find an internal diagram so don't know if taking the supply from outside the boiler will or will not by-pass and fuse.
In the main most boilers need a permanent supply, this allows boiler to cool once the boiler is turned off, and since it warns against fitting a time clock on the supply I would assume this is true of your boiler, so one needs to ensure the plug can't in error be removed while boiler is running, so normally one uses a FCU as then it is unlikely the boiler will be unplugged to use socket for anything else.
The heat link is class II so does not need an earth, however any installed cable should have an earth, so there is provision for an earth for when the heat link is used to supply the thermostat, since your heat link does not supply the thermostat the earth is not required on the heat link, but it is required on the boiler, and it seems it has been cut off short.
I have Nest Gen 3 and it is a bit useless, as it only measures temperature in one room, I still use it as it allows me with two wires to keep the thermostat battery charged and send info as to is boiler should run either for DHW or CH, but it is on my to do list to fit a proper thermostat that will link to the TRV heads. But my boiler is a simple on/off, yours is a modulating boiler, so really you do need the proper controls, not a lash up using Nest.
