How to preserve vitamin of fruit juice when making fruit jelly?
Take either some of the juice or some water (or other liquid), heat it up and dissolve the gelatin in it, cool to lukewarm and then add the rest of the juice, pour into the mold, etc.

I'm recommending this based on a method I used to use for making probiotic gummies with water kefir. The heat would kill the probiotics in the kefir if you melted the gelatin directly in the kefir, so using water/juice to melt the gelatin and then afterwards stirring the kefir into the dissolved gelatin mixture preserved some of the probiotic benefits.

To be more precise, you'd need to check the heat-stability of the particular vitamins you are concerned about and then ensure that the temperature of the gelatin mixture is lower than that temperature when you add in the juice.