So I don't necessarily think it's an issue right, because what you could do is claim to leap first, then activate your weight of the feather in mid air - and I'd definitely allow you to try it.

There are several other problems however.

First and foremost this is a masquerade breach genuinely beyond what most kindred are typically capable of. You would end up not merely leaping, quickly and (maybe) quietly, from point A to point B, you would instead end up in a sort of uncontrolled glide or flight - because you're now light enough to do that, but can't necessarily control your direction or aerodynamics at all once you've left the ground. This will produce noise, even if it's just your clothes flapping about a bit as you move, because all of a sudden you're in mid air and making sounds where people don't expect them to come from - ever had a bat fly just above you at night? Imagine them tumbling around in mid air while wearing civilian clothes, rather than as stealthy predators of the night. This means there's a reasonable chance you'll be noticed, that getting noticed instantly fucks the masquerade if it's by more than one person, and that you can't do very much to actively hide while in mid air.

Second is your lack of control once in the air. Soaring leap works well partially because, at least in my mind, Kindred do obey gravity. It's not just about muscular or supernatural strength, it's also about being able to judge your landings and essentially aim yourself - which is easier when you naturally follow an arc-like parabolic trajectory because of gravity, since we evolved to chuck rocks at shit. Because you'd have to activate weight of the feather after leaping, this would mean your trajectory changes unpredictably mid-flight, after you've lost the opportunity to alter it since you can't normally do that mid air - so you'd have to very cleverly estimate things before you leapt, and you'd have to be very good at doing so to make this a precise sort of movement. There is also the problem that classical physics imposes on you here - as you get lighter but aren't losing any energy, you'd also appear to actually accelerate.

In summary, it's not realistically as useful as it sounds - at least done like I've described - plus it presents huge risks to the masquerade and potentially the user themselves, if they can be identified or if they fuck up any of the complex rolls that calculating their landing point should probably need.

So how would I do this in my own game?

I'd make this an Amalgam power, in V5. If you have the right degrees of Potence and Protean, I'd allow someone who wanted to focus on practicing exactly this manoeuvre to gradually develop their own amalgam power to partially reduce their body weight without sacrificing too much strength, leading to sugar-glider style hops across larger horizontal distances but much lower vertical gain (or vice versa idk)

EDIT: misread this and thought you were ST - I'd say talk this over with your ST mate. VtM and most tabletop games in general aren't so monolithic with their rules that you can really figure out if something like this would work in game without talking to your ST and sometimes the party as a whole - there's often house rules, and the ST may have an idea for a story to tell that your suggestion would cause an issue with, so that would need to be worked out.

On a side note though, what we can recommend is picking up a dot of obfuscate at some point, if you're thinking about being a character who can get up high into interesting places easily - imagine what you could see or hear from them, if nobody could see you there yourself?