Hi Crosseyed,
You do not need to use a vaillant cylinder, not sure where you got that idea from!
Obviously Vai;lant will illustrate use of their own product in marketing
The vaillant VR10 probes fit beautifully into any 6mm (general standard size domestic) thermopocket such as megaflo or similar.
If you are using a traditional open vent copper/stainless cylinder (where you are unlikely to have a thermowell) the VR 10 can be inserted in between the polyurethane lagging and the cylinder wall. Cut a small square out of lagging at bottom third/quarter of your cylinder poke VR10 into place against cylinder and replace the lagging square.
You are right that the documentation of the Ebus controls is (and always has been) diabolical (in fact all their documentation is poor). However the controls are enormously flexible and capable controls that put many commercial systems in the shade for very little money (relatively). Unfortunately the vast majority of UK installers are too stuck in their ways to learn anything new and dismiss Ebus as too complicated
Its really not complicated at all, it's just not a Sundial Plan!
Just to clarify an earlier error on this thread: the VR10 (grey cable) is the standard temp sensor for domestic heating and hot water, it is used throughout the Ebus control range. The one exception is the VR11(black cable) wide range,high temp sensor which is used exclusively as a solar thermal panel sensor (yes - the ebus controls will integrate control of your panels and multiple solar loads! pools, cylinders, thermal stores etc)
The outdoor sensor is a (VRC693) is ready housed in a small grey plastic housing for wall mount and cannot be confused with the VR10/11
There is another outdoor sensor (VRC9535) but it is not sold in the UK as it integrates a time signal antenna for a signal not broadcast here in UK
To address your initial point: If you need to access the twin boiler stats in an ecotec plus boiler and don't want to use ebus controls this is readily done by using the RT or 345 switched live terminals( terminal designation dependant on boiler age) and then the C1C2 plug on the boiler wiring loom to provide volt free demand for DHW via a regular cylinder stat and zone valve (valve orange and grey wires Straight to C1C2) you can even buy a plug and lead for the C1C2 connector if you want though I normally cut plug off loom and use chocbloc/crimps/barrier strip cos I'm cheap
This will give you priority hot water with boiler flow temps set differently for DHW demand than a space heating demand.
If you want to keep heat demand ELV then newer boilers have a 24V space heat demand available.
As regards opentherm - as you say Vaillant make the VR33 opentherm adaptor which can be easily purchased online from the NL - many Honeywell Evohome users use the Evohome OT controller and VR33 to good effect. There's much debate on the Home Automation forum if you're interested.
Ebus far far pre-dates the arrival of OT and is streets ahead in capability and flexibility, can be configured to control tens of mixed circuits, multiple cylinders/stores, solar thermal, up to eight heat sources in cascade (gas boilers or heat pumps) and integrates with app control and the post install zoning Ambisense rad trv body sensor/motors (akin to Wiser/Evohome) and all the usual crop of digital assistants.
OT is essentially a peer to peer system (though certain commercial controllers like HW SDC Smile and Remeha/Broag etc do extend it) and its only really Honeywell (An OT member) that have made any real effort to market domestic OT stats here in the UK Also despite it being a supposedly 'open source' standard many controllers run into problems with incompatability with temperature protection/anti cycling limits in connected boilers that prevent the OT demands being satisfied (Ideal (who did produce some native OT boilers) have a particular issue with HW controllers) AFAIK OT really hasn't gained traction here, probably for similar reasons to Ebus and what OT native equipment that was on the market a few years ago is disappearing.
If you want any help sorting out best options for your system PM me
Simon (30 years an Ebus installer!)