From https://www.indy100.com/politics/jd-vance-dolphin-search-history:

Back in February, long before he was a potential vice president, Vance shared a screengrab from a X/Twitter account called 'Crazy Clips' which showed a video of a woman getting attacked by a dolphin. The post read 'woman gets violated by a dolphin and enjoys it.'

Now apart from the clip being a bit weird, especially for a notable US politician to be sharing, the big thing that people have picked up on is that the words 'dolphin' and 'woman' were highlighted in the tweet.

Given how X/Twitter's search engine works it is therefore likely that Vance, or someone else, had searched for these specific words in order to find the post.