I have all 6 Mata masks in both silver and gold, as well as the chrome Hau. I've also got that ridiculous-looking sparkly Avohkii that came with Takanuva and the gold Ignika, if they count.
IIRC, I have like four Vahi (two gold and two orange) and one or two Disks of Time. Think I got most of them in those parts tubs way back. Aren't Vahi fairly cheap on Bricklink?
Eh. Like a buck or two. Oh, don't you worry, I'll be buying one soon. I mean to say I wish I had one growing up. I covetted the Vahi as a kid. It's funny, because the Vahi's powers in-story aren't super amazing. It can slow down and speed up time, not not reverse it or allow the wearer to travel through time.
But, it's very use could RIP THE VERY FABRIC OF REALITY APART!!! So Tahu uses it once, nothing goes wrong, doesn't even help against the Kal... Yeeeeaaaah, kinda cruddy. Looks cool, though.
For such a coveted mask, especially during the earlier years of Bionicle, it was pretty useless. Makuta only wanted it to speed things up.
I just realised, too, every single time the mask has been used it's never been to any positive effect. At best Tahu delayed the Kal enough for Gali to go "Oh hey why don't we..." and Vakama maybe gave Lihkan an extra second to run up and take Makuta's blow, but neither times did the Mask's use solve the problem. In fact, neither seemed like it would accomplish anything anyway. (Granted Tahu didn't know the Mask only slowed time) Really Makuta's plan seemed like the best use of the mask. Fuck it. I'm making up better powers. The second I get that mask, Tahu's his own grandfather.
Vakama may have believed that Vahi had some other power than slowing just slowing time and so he tried to use it against Makuta, failing spectacularly. If he didn't have the mask, he wouldn't have just stood still in front of Makuta's shadow hand waiting for the Mask to do something, like he did in the movie. That would have meant that Lhikan would not have needed to step in between the attack to save Vakama. Damnit Vakama, you killed Lhikan!
What they need is a mask that blows shit up when the wearer points their finger at targets and yells "bang!"
Like, a Kanohi that makes Bohrok? Now the Kanohi that makes Kanohi that make catapults that fling Kanohi that make catapults would be called tha Kanohi Wiiljahk. Great mask of call a paramedic.