Will Matrix V-light turn my medium brown hair to dark blonde hair?

I'm going to assume you are a stylist in which case I'm wondering why you are asking this question. V-Light is a lightner and it can be mixed to different strengths depending on how light you want to go. However, you won't get a nice shade of blonde just by using the v-light. You'll have to use the v-light to lift your hair to the level you want say a level 6-7. Once you wash that off you will be a nice shade of orange or gold. At that point you apply the blonde shade most likely with an ash base to tone it. You could use v-light alone for just hi-lighting without having to tone, but not all over color-it just won't give you a nice shade all on it's own. Hope that helped

Will Matrix V-light turn my medium brown hair to dark blonde hair?

It depends on the color dye you bought.