The idea that he is completely fictional is ludicrous. Here's an interview with him in which he clearly exists. Unless you believe the battle of Stalingrad and World War II also never happened, there is no reason to believe that Vasili Zaitsev wasn't an actual person.

I'm presuming that what you're actually asking is how much of his story is based on fact versus propaganda. There's no reason not to believe the basic premise. We know that there was a battle fought in Stalingrad (known as Volgagrad today). We know that snipers were a feature of warfare during this era. We know that some snipers were extremely effective and in some instances capable of inflicting hundreds of casualties. No reputable historian would debate any of this.

What is debatable are the details. "Body counts" are hardly an exact science. They are subject to all sorts of bias, padding, and propagandizing. Furthermore some aspects of his story can be safely dismissed as myth. There is no documentary evidence that his duel with an ace German sniper named Major Koenig ever happened. Additionally any books or movies about his exploits should be taken with a huge grain of salt, as by necessity they are largely based on speculation and imagination.

However, to think his entire existence was fabricated is more fantastic and less believable than the history as it stands.