In the Dragon Ball Super anime series, when Toppo used hakai he had to turn into God of Destruction mode. Now that Vegeta has been taught hakai, does this mean Vegeta now is able to turn into God of Destruction mode just like Toppo, or he just can use hakai?
Toppo was able to use the God of Destruction Mode because he is the candidate to be the next God of Destruction.
Vegeta isn’t a candidate, so presumably he cannot make the transformation. Goku also isn’t a candidate for God of Destruction and he was able to use Hakai against Zamasu:
After achieving Perfected Super Saiyan Blue, Goku is able to perform the Hakai to a limited degree, however, it's much slower than Beerus', destroying Fused Zamasu progressively and requiring concentration throughout - thus Fused Zamasu takes advantage of the weakness by using Future Mai as a human shield, stopping the technique.
Everything indicates that while Hakai is a signature move for Gods of Destruction, it’s not exclusive to them or candidates who can use God of Destruction Mode.