Yeah, Vassell's defensive upside is much higher than Haliburton's. Haliburton is a good help defender but his body is unlikely to ever allow him to be a good individual defender. Vassell has a chance to be a great help defender (like top five in the league great) and a really good individual defender.
Haliburton's offensive upside is higher than Vassell's in all aspects other than possibly as an isolation scorer. Vassell, due to his high release, might have higher upside as a three-point shooter ... but Haliburton has more range so it's probably close to even.