OK, here goes..original recipe calls for 2.5 Tablespoons. That would be 7.5 teaspoons ((2x3) + (.5 x 3)) = (6+1.5) = 7.5. Now, the best divisor would be 8th's. Just trust me on that one, unless you want a REALLY long post. So, you have 7 teaspoons divided into 8ths, which is 56 8ths. Then you have to add the .5 teaspoon, which in 8ths, is 4. 56+4=60. So the origional recipe calls for 60/8 teaspoons. Stay with me here. Now you want to divide the recipe into quarters, so divide 60 by 4, which is 15. This gives you 15 over 8 (can't figure out how to make my keyboard display the fraction) which is an improper fraction. To reduce this fraction to the simplist form, divide 8 into 15, which gives you 1 with a remainder of 7, or 1 and 7/8' s of a teaspoon. But the others are right, at this point, anywhere between 1 1/2 and 2 would work...depending on the recipe and what your are measuring. Since you are measuring vanilla I assume it is for a cookie, cake or cheesecake and it probably wouldn't matter that much. Take a look at exactly how much goes into 1/8 of a teaspoon sometime.