Unfortunately stopping the mouse button click from getting passed through to other applications also prevents reading its state (this is a Windows API restriction, not VoiceAttack specific), which is required for detecting a long press, so the closest you can get using a mouse button is to have it click on short press and click and press R on a long press.
If you have a mouse that comes with software that allows you to rebind the right mouse button to press a keyboard key, that would allow you to use the "When I press keys" feature instead, the underlying Windows API for which does allow both blocking passthrough and reading the state.
A rarely or never-used key would be best for that; The F13 to F24 keys, which aren't physically present on most modern keyboards, would be ideal, but not all software allows you to map those.
You can try having VoiceAttack press one of them when your mouse software asks you to press a keyboard key to assign it to a given mouse button (presuming that is how it functions).
Topic: How to make a long press command? (Read 2005 times)
VoiceAttack User Forums