I haven't delved very deep on Vocal. I just took a look at some stuff that they had picked as Top Stories and Our Favourite Writers, and saw some absolute cack. Secondary school level condescending pat on the head and 'well done!' with a forced smile from the teacher. (Miaow!)
In those circumstances I tend to make a snap judgement: these people's opinions are worthless. But then they picked me, so they obviously have some taste. (Joke.)
It just doesn't seem like there is the rigour 8ahem!) of the boost/curator set-up - just randomly picked stuff. But Maryan Pelland says about 1 in 3 of her non-fiction posts copied over from Medium are getting picked. So it's maybe worth doing. I wouldn't pay for membership to get the right to enter the prize prompts until you get a feel for it. According to another commenter, they are an absolute pain if you want to cancel.
But just copying across a bunch of Medium stuff to a free account is probably worth it.
You can get the fiver for Top Story picks, and also earnings for reads.
So far the only comments on my stories have been 'Great post - check out my profile' stuff, so no genuine readers yet. But I've put zero effort into engaging with the community there. I just don't have time, and at first glance, it is poorer quality writing than the top level of the feed and recommendations here (seriously).
But then I'm a picky and arrogant curmudgeon, so ignore me!