That’s interesting because my vac scheme last year had a relatively low conversion rate (around 50%) and they’re generally known for being lower than other firms. It obviously depends on the individuals on the schemes.I did a VS at Slaughters last year and I'd say that their conversion rate is pretty high. Only 4 people out of the entire 20+ on my VS didn't get the TC. One thing I will say is the TC interview at the end of the vac is the exact same as the normal TC interview direct applicants do.
Although you're guaranteed the TC interview, you are expected to re-submit an updated CV and a CL through their application portal. (you don't have to change anything if you don't want to but I guess there is an expectation that you will since you now know more about the firm after the VS). And you do the one hour case study just like direct TC applicants do.
Because the scheme is unassessed (i.e. you don't submit work throughout the VS that gets marked by anyone) most of the decision on whether or not you get a TC is based on the case study and final interview. So as long as you perform during those and don't have glaring attitude problems or anything during the scheme, I'd say your chances of conversion are p good.