As per Twitters Terms of Service you can not directly send a Tweet to a new subscriber with a third party app like Zapier but you can among other things add a user to a list.
Assuming you already have a Twitter account integrated into Zapier and your VAP Trigger is setup it is simple enough to get your Zap setup.
Choose App and Event
As you go to test your Zap it is important to understand that there is an Anti-Spam filter in place.
What this means is a Phone Number will only trigger one time when entered into your opt-in box.
So if you wish to test your Zap more than once you will need to either use different phone numbers to enter into Opt-In Box or set up a different Video Project to test with.