There are two methods to finish vehils quest.
The 1st one: This is simple, just use the magical stone twice in the slums area. To simplify it more, use the stone first then return and discuss the slums with alioch; sleep then let him recharge the magical stone for you. Repeat it again, and then you will be ready to save her. (Don't forget to take the potions with you before rescuing vehils.)
The 2nd one: This is also simple, but a little bit longer. Visit the affluent part of town, (The one near the slums.) you will meet a woman named priscilla (the mayor.) she will be arguing with someone, but that doesn't matter, what matters that you choose the right choice and compliment her. She will immediately spill the beans and tells you about jasmine's whereabouts. Now visit the barracks and investigate jasmine's quarters. Return to alioch and save vehils. (Remember, you have a chance to save her until day 5, after that its game over.)
I hope this clarifies and helps you with the quest.^^