VPN + HTTPS = 100% anonymous?

Twitter doesn't know the user

If you have ever used that browser to connect to Twitter outside of the VPN then it is possible that twitter have used cookies or (even in the case of a complete browser data wipe) browser fingerprinting to identify you. Even if they haven't you should assume their ad providers have.

no one can know who is the user, except the VPN provider.

Anyone with visibility of entrance nodes and exit nodes to the VPN (ISPs, state actors etc) can apply packet matching techniques to try and identify traffic flows.

You also have the risk of both Twitter and the VPN sharing the information they hold on you with other parties.

As Steffen points out, you are perhaps more anonymous that you would be without the VPN, however you are far from 100% anonymous.

• Your web browser itself can reveal a tremendous amount of information about your computer, browser and other services you might be connected to. JavaScript has been used to de-anonymize people on Tor.
• Things you post, and interactions you have while on social media can potentially be correlated back to you, especially if your OpSec is weak.
• A DNS Leak could unmask you
• If your VPN provider keeps connection logs and operates within a jurisdiction that would require them to turn over that information (or if they wilfully comply with LEA's) then you could be hosed.
• If you slip up and land on the radar of a Law Enforcement Agency they can look up whether you had payments to your VPN provider, engage your ISP to correlate traffic from the ISP to the VPN to when posts were made on Twitter, etc

I don't know if you can ever be 100% anonymous on the internet, but if you can it requires more than HTTPS and a VPN connection.

Use of an VPN only means that Twitter can not determine details about the user from the IP address. It might though have other ways to get enough details about the user, for example from cross-domain user tracking (using third-party cookies and other techniques) which many sites employ.

Apart from that Twitter might determine that the IP address belongs to a specific VPN. And, if you broke laws while interacting with Twitter they might use the law to require the VPN to give detailed information about you. If the VPN provider keeps logs which user was assigned which IP address at which time they will probably provide these information to law enforcement too.