Okay, I'll try to give you some useful tips here, because I had the EXACT same problem. Up until like 7-8 months ago, my leg was like this:
(ignore the fact that that whole picture just screams bad rider hahaha)
I was never able to tie my stirrups to my girth because I could never find a way that wouldn't irritate Danny - he's the most sensitive, thin-skinned horse on the planet. I did, however, have the luxury of someone on the ground CONSTANTLY nagging about my leg, and also showing me, at a halt, where exactly my leg should be. If you have someone do that it feels sooo weird, like your leg is at the flank, but that's where it must stay! Here's where awesome leg muscles and good willpower come in handy.
My trainer has a trick that we call the "mental checklist." Really, it's not a trick so much as an automatic thing that every rider develops over time, but it really helps if you think about it - when you're riding, just make sure every part of your body is in the right place at once, all the time. Study and visualize the way you want your position to be. Think and obsess and watch videos of your riding as much as possible.
It could be that Vega's bucking is caused by the leg - when your leg is out of position, so is your balance. I know a LOT of problems went away when I sorted myself out.
Sorry I couldn't give you a simple tip - this is just what worked for me, and I guess it's kind of similar to using draw reins versus engaging the hocks - the first one leads to an artificial headset while the other creates a truly "on the bit" horse. Did that make sense?? What I'm getting at is, if you tie your stirrups to your girth you may find that the second you untie them, your leg will slide forward, but if you "teach" your leg to stay in the correct position, there it will stay.
I hope this helped! Good luck!