Thank you for the question.
This is a known issue in Q1 2008 SP1 version. However, it is already addressed in our latest Q2 2008 SP1 version and you should be able to see the separators in the Vista theme. Please download this version and give it a spin.
In addition, please note that in Q2 2008 we introduced a new approach for working with the predefined themes. For more information, refer to the following Knowledge Base article: Managing predefined themes in Q2 2008
If you have additional questions, feel free to contact me.
All the best,
the Telerik team
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