So I inquired at Viessmann, the Vitocal 333-G is still available but that wasn't the point. However, the internal cooling was apparently discontinued due to lack of demand because the internal cooling cannot be combined with an external buffer tank. Now there is only the Vitocal 333 G without cooling and then you just have to buy the external component...
I'm more concerned about my negotiating position and my rights. If I now say "no, I want a Viessmann" and my installer says it's no longer available or I have to pay an extra €2,000. Where do I stand then? Am I allowed to insist on what I signed and he has to cover the additional costs or am I the one screwed who either has to do without the cooling, get another manufacturer, or pay the surcharge?