You are mixing stuff that has units (ko, because you calculate it using built in constants, which have units), and stuff with no units (your data). Therefore PhQfit(vx,Beta_guess) has units, but vy does not. The two must have the same units. Actually, I'm a little surprised that genfit does not complain. Either don't use units at all, or use them consistently. The latter is by far the best option.
