I don't think most of Trump's actions are done out of vanity, but I feel like this one is. It also delineates an exact narrative for Republicans to believe

All he cares about is anticommunism, bravely pardoning criminal presidents, doing good wars (but not bad ones!), expanding the surveillance state, and beating literally all the democratic presidents. Economic downturns during republican presidencies aren't their fault and upswings during democratic presidencies are coincidental

To humour himself. Trump has a massive ego, even by US president standards.