new SortClause(VConversationmatchFields.Count, SortOperator.Descending)
should be
new SortClause(VConversationmatchFields.Count, null, SortOperator.Descending)
(as adapter uses different ctor, as it needs separate persistence info)
you can also do:
VConversationmatchFields.Count | SortOperator.Descending;
which is preferable, so you don't need to mess with ctors.