The three-year-old joins Mullins from Andrew Oliver, but not as a potential jumper and more with major Flat races in mind.
Gustavas Vassa has had just three races to date and the bookmakers are taking no chances in making him favourite for the feature handicap.
Harry Herbert, racing manager for Highclere, said: "He's an exciting horse that we bought thinking he could potentially be a quality staying horse on the Flat, maybe even a Melbourne Cup horse for next year.
"We'll obviously see how he goes, but we certainly bought him as a Flat horse rather than a jumper. Willie is very happy with his work and this looked a good opportunity to get him started."
St Stephens Green will be aiming to give Emmet Mullins, a nephew of Willie, his biggest win since turning his hand to training at the age of 25.
A smart dual-purpose performer, he will be heading back over hurdles at some point.
"I felt he ran better than his finishing position suggested on Champions Weekend (10th), he was only beaten two lengths, and he probably found that trip a bit too sharp as well," said Mullins.
"I'm hoping for another bold show and hopefully we'll get another good run out of him. After this we're thinking of one more run before giving him a break when you get your real winter ground.
"Whether that is over hurdles, in something like the Greatwood, we'll wait and see. We'll then bring him back for a spring campaign.
"There's more to come from him hopefully. We only really got his jumping sorted out when he won at Killarney, but it just so happened that there were races for him on the Flat so we switched back."
The only non-Irish runner is Brian Ellison's Nietzsche, another who will be heading back over hurdles after this.
He finished third in the Fred Winter at Cheltenham and had a spin on the Flat recently.
"He's in good nick and should run well," said Ellison.
"We gave him a nice break after Cheltenham and then he had a run at Doncaster last month. He'll be a lot sharper for that and I think the fact it is a flag start will help him.
"It will be back over hurdles after this and we'll head to Cheltenham, I'd have thought, for the Greatwood. He was unfortunate at times last year, he kept bumping into one."
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