When you accept from an enhanced listbox immediately before, or in the middle of, a symbol, Visual Assist replaces the text to the right of the caret unless that text is itself a known symbol. To change this behaviour, edit the registry setting:
HKCU\Software\Whole Tomato\Visual Assist X\<IDE spec>\CompletionOverwriteBehavior
0 = default behavior: VA will overwrite text unless it is a known symbol
1 = never overwrite text
2 = always overwrite text
Close all instances of the IDE before editing the registry to ensure your changes are not overwritten at IDE exit.