Both teams aren't in top form.
Vitality lost in Global challenge while they were the better team, they just ran out of steam, were tired (astralis too though), they 16-2 on the first map and couldn't close out Dust 2 despite ZywOo going nuclear, he had to convert the ace after doing 3 deagle kills in eco^^ if not it was almost impossible considering all the individual mistakes from everybody (all 5 players).
Astralis can play great and win even when they're not in form, they won Global challenge (of course they just started to play as the usual astralis in August so they weren't as tired as Vitality but still).
It can go either way here, from a tactical standpoint, no team is more refined than them in my opinion. And the individuals in both teams are really versatile, roles are handled well.
I'd like to Vitality win but either way I think we're going to have the teams in the grand final if this isn't Na'Vi one tournament's run of this year^^. (Starseries in 2019, Katowice in 2020, which event will it be this year, though I expect good things from them, I don't think they've done any bootcamp).