Could you clarify: Are you executing a command that presses F1, or are you pressing F1 to execute a command within VoiceAttack?
If you're executing a command that presses F1, make sure the command target is set correctly. If it's set to "VoiceAttack" in either the main window(the "Send commands to" option), or in the command itself(the "Send this command to" option), change it to either "Active Window", or the window title of the game you're trying to send the command to.
If you're pressing F1 to execute a command, make sure you have the "Do not allow key to be passed through" option checked in the "Command Shortcut" dialog(opened by clicking the "..." button next to "When I press keys" in the "Edit a Command" window.