I do mine by feel/eye. I use cold water to mix the plaster( if you use warm water the reaction will start too soon). You don't want the plaster to be too liquid but you don't want it too stiff either(as you already found out).
I don't use vasiline because I find it too thick. I use cooking oil. it soaks right into the plaster so you don't loose any detail. the plaster still separates very nicely.
"Painting" the bait with pop as it sets in the first part of your mold will help with the bubbles; just don't put any oil on the bait itself. A lot of what we call bubble impressions are actually water drops formed in the plaster by too much release agent. We only see the resulting hole once the water is soaked into the plaster. (Remember, water and oil don't mix).
Also, when you pour the pop into the mold, make sure you do it slowly from one corner and let the plaster flow over the rest of the mold.
www.novalures.com