Responding to posts is fun, because I get to think about even my own responses.
Your system was designed to collect solar energy specifically to use the hot water. Then, you had the idea of making good use of the hot water. OK, fine, good question.
Now, the problem is, we need to do the whole system backwards from the way the hot water people are selling it.
I can tell you that what you call vacuum tube solar system, over here, is known as evacuated tube solar heaters. There is a U.S. government document at one of the energy labs { you have to find it yourself, but it is there, on public web site } that looks at these and the maximum temperatures they can achieve. They can easily reach the required temperatures to power a Stirling engine. And, this is without boosting them with trough reflectors, etc.
Instead of using the evacuated tubes to heat water, trying to make sure not to create steam, use a different working fluid, such as an oil, in bascially the same type of flow and storage configuration.
The oil storage tank will drive a Striling quite nicely, hold heat throughout the night, if you make it large enough compared to what the engine uses and how well it is insulated, and ....
Rather than try to get a Striling to run from your hot water system, you can easily get hot water from a seperate run of pipes going through the hot oil tank!
The evacuated tubes work especially well in overcast areas where photovotaic do not work so well, too, such as the Netherlands, and much of Europe.
As I would love to live in the Netherlands, if you have a major company that would hire me, I would be glad to change the energy equation of Europe for the mere price of a job until I can retire!
*huge grins*