The router's update client software updates dynamic DNS hourly. Even if no IP address change has been made by ADSL provider. This must be wrong.

One dynamic DNS provider refers to this behaviour as "abuse" and will block offending update clients and suspend the user's account.

I am not yet aware if my provider dtdns.com will take this approach too.

Have others experienced this problem? Do all Vigors behave like this?

My firmware is not the latest available. I have downloaded the latest v2.5.6 (Release) from the Draytek site but the documentation is confusing.
The readme file refers to 4 files in the release including v2k6k910.all and v2k6k910.rst (UK Firmware) pair of files that I should use. However only 2 files, v2k6k256.all and v2k6k256.rst are in the zip file.
These do not appear in the documentation. So I have not tried an update yet.

re: Dynamic DNS - hourly updating of DTDNS IP
I have asked DTDNS about this. At the moment, they have no policy restricting updates.
This issue might only affect the DTDNS update module.

I also tested the NO-IP module on this router. It does NOT update hourly, only when changes occur.

I also have a Vigor 2800V and this does NOT update any provider hourly but behaves as I would expect - only contacting all DNS providers when my IP address actually changes.