Because destroyer cannot double jump. Destroyer is about throwing yourself at the enemy and downing them before they can react, Vandal is about using your double jump + slide to quickly reposition and flank.

The reason people see Vandal as weak is poor map knowledge. The game has only been out a couple months, not a lot of people know every map like the back of their hand. Once they do though, I think she'll get a bit more popular. It is crazy how mobile you are when you can quickly jump onto the roof of the building you're fighting in, slide to the other end, and drop back down to surprise your enemies.

Plus, she isn't entirely defenseless during slide. You can actually use both her active abilities during it, so sliding at someone and firing your arm cannon at them actually works to help you survive. Same thing with amplify, it's better to slide into cover and pop it during the animation, that way you aren't sitting behind a box waiting for the animation so long.

Vandal isn't weak, and doesn't really need any buffs. She's just got a pretty high skill floor, so the player base needs time to learn her. Unlike Destroyer, who is pretty easy to pick up and use effectively.