You didn't mention which version/bitness you were running, but I did a clean install of VoiceAttack v1.8.9 of both bitnesses, installed vJoy (downloaded using the default green "Download" button on
https://sourceforge.net/projects/vjoystick/), installed the plugin for each respective bitness, and ran the "test button 1" command, which joy.cpl showed being pressed and released correctly.
I also swapped the bitnesses of the plugins and their VoiceAttack installations (x64 plugin for x86 VoiceAttack and x86 plugin for x64 VoiceAttack), but that results in the plugin not loading at all with a completely different error message.
Do you have a screenshot of the error message you're seeing?