Couldn't this account be closed and tracked as well ?
Also, it gives an email-ID for correspondence, are VPN's so secure that this mail account origins couldn't be traced?
Could it be closed? Absolutely. Twitter can close any account they would like and the account owner can do nothing about it short of moving their content elsewhere. However, tracking the account creator is a different matter. If they used an anonymity network like Tor, and sandboxed the browser or otherwise protected themselves from the potential of browser exploits, then they would be almost impossible to track. Actually tracking them would either require using very valuable 0days (which may not even be enough, depending on the level of precautions taken), or by hoping the hacker slips up his OPSEC.
It surely can be closed by Twitter once they get to know that an account has violated the terms & conditions of holding an account with them but there are hundreds to thousands of such accounts ranging from hactivism to cybercrime and they use Twitter as a platform to spread their message. You may try doing report abuse!
With regards to tracing, in the current times it's really hard to trace real source with different anonymous services and VPS services being offered globally at throw away prices. This is the reason, attribution in the current days has become more complex and one may do this only by following an actor and their Indicators for significantly longer times.