It's a valid market While the assumptions of the challenger can happen, (a twitter account pretending as Vitalik, other ways to mention the word 'Omen' without referencing to the official twitter handle etc ) the market's question is understood as referencing to the official account of the well-known ETH founder Vitalik and Omen pertains to the decentralized prediction market where this market is posted. It'll become tricky if it has to specify the rules of the tweet mention.

This market is unlikely to be resolved as invalid In this context about means "Concerning; with regard to; on account of; on the subject of; to affect" https://en.wiktionary.org/wiki/about definition 4. Using the Omen resolution rules (https://omen.eth.link/rules.pdf) "Entities are assumed to reference the most obvious entity with that name, taking the context into account." This question is asked on the Omen prediction market platform, so taking the context into account, it refers to Omen, the prediction market platform. So if Vitalik uses the word "Omen" but without referring to "Omen, the prediction market platform" it would not make the market resolve to Yes. The question asks about Omen, so prediction markets, in general wouldn't be sufficient. Using again the same rule "Entities are assumed to reference the most obvious entity with that name, taking the context into account.", the only famous and obvious Vitalik Buterin is the creator of Ethereum.

Challenge Justification What does "about" mean in this context? Does Vitalik need to tweet @ the Omen handle? If Vitalik tweets the word omen outside of the context of this market platform, is that sufficient? If Vitalik tweets about prediction markets in general, is that sufficient? If a non-famous person named Vitalik Buterin tweets any of the same, valid? Certainly invalid.