The idea of painting a vial to look like it's filled with fluid does seems like a scary thought… but it's just another painting obstacle that can be overcome with a little practice and a firm understanding of the principals that make it work well visually.
Initially when I set out to make a bunch of Dark Eldar Coven models I was scared to death about all the vials I would have to paint on Haemonculi, Pain Engines, Wracks, Grotesques, Scourges, etc. I developed a 4 step process that I would consider High-Quality Table Top.
Is it Golden Daemon? No, certainly not. But it does look great on the table and is realtively quick and effective process.
Step 1-2 : The Base Colors
This it the MOST CRUCIAL part of the whole process. You have to understand the process of how liquid moves in a glass vial and paint accordingly. Using the horizontal edge of the minis base as a guide, imagine how the bottom half of the vial will look once filled half way with fluid. It's crucial for each vial to have the top edge of the liquid parallel with the base edge. This will vary from vial to vial depending on how it's positioned on the model.
Step 3 : Fluid Highlight
All you do here is simply highlight the upper edge of the fluid line. The trick is to make the edge where the fluid meets the empty glass hard. The edge where the fluid highlight meets the darker fluid needs to be blended a bit. This makes it seem as if there's more depth to the bottom of the fluid filled areas.
Step 4 : Vial Highlight
The final step here is to paint the vial highlight. A simple thin line down the upper outside section will usually suffice. I prefer to use a light grey instead of white for this as to not overpower the effect of the vial painted underneath.
Optional Step 5
You don't need to do this, I usually don't. But you can paint bubbles into the fluid, by simply adding some tiny dots of a lightened up version of the base fluid color.