the best option is sSuke because they appear in all or other that you want kakuzu itachi edo .. in greate plates you win 16 frags 500 keys 80 / pots training.refine epic fraga d emuy good ninja s. and protection and experimental vials around 90 to 120 of the two. Using simplement 3600 cupons

Just ignore him - he likes to spam the forums telling people to get sasuke rinnegan and do great plates without any good context.

As for your question, you are bound to get free protection vials and breakthrough vials without even trying but the amounts are low. Since it not guarantee to get a certain skill, try to get a lot items. I usually try for 150+ protection vials and 200+ breakthrough. As a f2p, personally I wouldn't spend my coupons as I could use my coupons elsewhere as I'm earning these items for free. If you are paying, go for it.

No, don't take them over training potions and command flag frags. Those two things should be your #1 priority to collect. Also don't pick advanced vials/protections over refines. The only thing I'd pick them over is Mt. Myoboku stuff.

Also, there's no real reliable way to get the normal protection vials from events. They are only in them rarely and there's pretty much no other free way to get them. It's so frustrating and a lot of people have asked them to put them in the moon scrolls shop but who knows if it will ever happen. But yeah that's just how it is right now.