Reasoning that if Vivienne really only wanted to test her potions, she could have simply told you they were normal drinks, you decide to take the blue potion. Blue as in blue raspberry, right? Hope, on the other hand, settles for coffee. Vivienne simply clicks her fingers, and another cup of coffee appears in her hands.
"Well, I'm certainly glad you've chosen to stick around, if I may say so. You're the first guests I've had in ages. And I do mean ages."
"How so?" asks Hope as you take a sip of your drink. It turned out to be more like cotton candy than raspberry. It's sickly-sweet, to the point you have to force yourself to swallow.
"Well, maybe not literal ages," says Vivienne. "But decades if not centuries. As you may guess from my demonstration a moment ago, age isn't exactly a concern of mine, although a misfired spell does keep me confined to the grey section of the woods you passed through. Fortunately--"
"Urgh...!" you groan, doubling over as you feel your stomach churn.
"Oh! That may have to wait, it appears the potion is taking effect."
"Well what's it doing?!" Hope asks frantically as you feel fire in your belly.
"Nothing harmful," Vivienne says unhelpfully. Unfortunately, the fire takes that moment to push its way out from your insides, quickly overtaking you entirely.
"WHAT DO YOU MEAN, NOTHING HARMFUL, HE'S ON FIRE!"
"It won't hurt. Well...not seriously."
From there on, the only noise you can make out is a rush of wind in your ears, and Hope shrieking louder still over it. And Vivienne was wrong - it does hurt, and seriously at that. Within the blaze, you can feel your bones shifting, grinding against one another, sanding off chunks that quickly turn to dust. Your skin bubbles, shifting as those bubbles pop and deform your flesh. Your teeth - no, your entire jaw cracks, leaving you unable to even scream, and for a moment, you feel your eyeballs boil and watch your vision fill with steam.
Then, as suddenly as they appeared, the flames vanish, and you collapse onto your side. The pain is gone, although a not-insignificant discomfort remains as you realize your form has been altered significantly.
"See? No harm done."
"I...wouldn't exactly say that's no harm."
You force yourself up into a sitting position, noticing a very distinct shift in how your body responds to your commands, not to mention the change to your weight distribution, now distinctly more focused on your chest and rear. You look down, and of course see the reason for that: breasts.