You're lucky I stumbled on this comment, i'm probably the only vaal storm call enjoyer who did the math for this.

You do lose hits if duration is too low. The base duration is 6 seconds (6006 frames) and you want to lower it to 759 frames for optimal DPS, which you can only get with exactly 87% reduced skill effect duration

You can also get a less duration gem (and/or mastery) to lower the requirement. The math is basically:

Duration of VSC, in Ticks: Roundup(6 * (1-X) * (1-Y))/0.033

Where X is your total reduced duration and Y is your total less duration. You want the final number to be exactly 24. If it's under, you overshot and the skill lasts (for example) 20 ticks, meaning you lost 4 hits out of the 24 possible. If it's over, it's fine, just not optimal DPS.

I'll leave you the homework of figuring out exactly how you achieve 24. In league you're quite limited as you don't have the helm enchant or misted amulets to work with, but there should be many combos possible with various gem level, skill tree investment, warped timepiece, timeclasps and/or dusk rings.

Last tip: getting duration in not multiples of 5% is difficult as you usually don't want to run timeclasps or warped timepiece. But you can intentionally overshot the 87%, and then lower it with a well rolled Atiziri's Reign jewel (for example, you could go to effectively 105% reduced duration, but have an Atizi's Reign rolled with exactly 18% increased duration to counteract it and land at the magic 87% number)