I know this is a super old thread - but I've been working on this recently - managed to make up a very close equivalent out of a 1k thermistor and 365 ohm resistor and 2210 ohm resistor.
The additional resistors are needed to linearise the response of the thermistor in the same way as the vaillant component, otherwise you get it matching at one point in the temperature range and miles off at the extremes.
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