I had the same problem. I had four Vivi Nova's and then got a Protank. Way different draw. Once you put the Vivi's down and mainly use the Protank,you won't notice it as much. Are you using the beauty ring that came with it? If not,try it. I think it did better with the ring when on my Ego Twist. But on my Vamo,it's awesome. Good luck.
I'm a cheap guy, so I've never actually bought one, because I just use a short piece of thin wall silicone tubing, cut to about 1/4" long (length will vary) and slide onto the battery 510 collar. Fit the tank, or any 510 liquid container, slide the tubing up to block off as many vent slots as needed. Ta da! instant airflow controller.
Protanks do have an airy draw, I like the easy draw but, it does need higher voltage, above 6 at least, 8 for clouds of vapor.
You might want to try using evod heads in your protank for a tighter draw. The two are interchangeable. Unfortunately they won't be around much longer, as kanger is switching to the protank style heads only.
Above 6? That seems really high.
Well I tried the tape method until I get home from work,,,,and it worked!!! It improved it big time.. Thanks guys when I get home I'll do the other method and cut some plastic.