May I ask why Vayne's ultimate has such a low cool down?

100/85/70

She gains so much with this one ultimate; bonus AD, bonus MS, and a chance at invisibility that is not easy to counter if they know what to do. It's like playing as Twitch only more unfair. At least Twitch needs 6 stacks to use his E; to deal only 290 base damage and 150% bonus AD or 100% AP. Vayne gets to deal 60 + 8% of their max health per 3 shots; that means after 6 shots, its 120 + 16% of their health in damage (for free as Twitch must use 90 mana).

Twitch's ultimate is sort of like Vayne's. Twitch's ultimate should remain at 100 second CD at its lowest but it also has a cost of 150 mana and the bonus AD is just 40 (which does not effect those behind as it loses damage after the target is hit).

Twitch was hit with some rough changes to help players cope with him, why is Vayne different? Vayne shouldn't have such low CD's on her spells and it shouldn't cost as low as it does if it does have a low CD.

Vayne's Q should cost 30/35/40/45/50 as giving bonus damage and a possible dodge/gap close is too powerful being both said and in-game.

Vayne's W can't be really changed so there's not much we can do here for now.

Her E even giving a possible stun is something scary on a ADC, especially one with a true damage passive, so this costing 90 mana is okay and I can't do much to it either.

Her R should honestly cost 100/110/120 mana. Why should it? Because of Sivir. (What?) You heard right, Sivir's ultimate. It may passively give bonus AS (that got nerfed recently) but its active costs 100 mana and gives only a burst of MS that lasts for like 3 to 5 seconds. Why does Vayne get a boost in MS that lasts 8 to 12 seconds? Let alone, she gains bonus AD and her Q that also gives bonus AD can now turn her invisible. Yeah, this one really needs to cost something.

Sorry if its a "because this champion is like this, this one must be like this" kind of post but I'm kind of tired of her having one of the weirdest and cheapest ultimates for a ADC while even now marksmen have more reasonable and even more expensive ultimates. If any ultimate needed to be cheaper, I would make Sivir's, Urgot's, and Kog'Maw's cheaper (Sivir's could be free and so could Urgots as they both don't deal any damage nor do they give any real damage boosts, but Kog'Maw's could start at 40 mana and then continue to add up.)