Which version of ViEmu and Visual Assist X are you using?
Running Windows XP SP3 I have installed ViEmuVS 3.0.6, and VA 1917. Sitting in a .cpp file in VS2005 I added a new line to a function with 'o' and typed:
std::string
the listbox shrank as expected, without any problems. This is using the default settings for both ViEmu and VA. What are you doing when you see this problem?