Hello!

As far as I know, the chances of getting a skill breakthrough or trial via Vials are being reduced each time you lock a maxed skill. Meaning that having 4 locked max skill breakthroughs will give you low chances of having a successful vial for the last one. For that case, it is much better if you will use Ninja Fragments to get the last skill, especially if the Ninja you're breaking is a Regular Ninja that can be farmed from the Elite Instances. Invest your coupons on better things like power-ups and such. Sasuke's regular version isn't that useful when you reach your mid-game stage since there will be much more useful Barrier Users than him. Still, it's all up to you because it is your game and not ours, we can only give opinions and suggestions here for you to have an idea. Thank you.