You might want to start with the Subversion.bld sample and tweak it a step at a time for your own use. Testing with VBP 7.0 and Subversion 1.5.5 on Windows XP SP2, the commit step in the sample works as expected.
http://www.kinook.com/VisBuildPro/Manual/svnsample.htm
Beyond that, it sounds like a question for the makers of Subversion. You can check the 'Log/Show command-line...' checkbox on the Advanced tab (Switches tab for VBP v6), then rebuild the step to determine the svn.exe command-line being used.