First of all you would need to Select your Occulus Quest (or your current VR headset) as your Input and Output within Voicemod Settings; this will allow you to use Voicemod within any application that uses a microphone while playing with your VR Headset.
In this case, once you are inside VRChat, go to Settings > Microphone and select "Voicemod Virtual Audio Device".
Take into consideration that Voicemod needs to be Open for this to work.
Go back to Voicemod and choose your favorite voice.