Hi there. I'm currently working on a Venus Square to improve my throat health, but...
During the longer chains of vibrations, I've noticed that my legs start to go numb (sometimes numb and pain) and my back starts aching horribly. That and my throat is sore afterwards and sometimes during the working.
Are there any tips on how to prevent this?
Well, most of this can be fixed with correct posture. If you're sitting on the ground either cross legged or in a lotus position the knees go below the hips. Otherwise they will go to sleep. If you lack the flexibility for lotus position, use a pillow or multiple pillows to lift yourself up to the point your hips are above your knees. Same with your back, keep it straight.
For the throat, its a bit harder, maybe try to drink something warm beforehand, or search online for exercises that singers use for their voices, though I had the same issue and it went away with time, the vocal cords get adjusted to the stress.