I’m not an expert in this, but I don’t think it would be very much. Isopropyl alcohol is quite volatile at room temperature, having a boiling point of about 83°C. Pure solid vanillin, meanwhile, would be just about to melt at that temperature, and wouldn’t actually boil until 285°C. There’s more to evaporation rates than the relative boiling points of substances, of course, but my guess would be that these are widely enough separated to make a room-temperature evaporation work pretty well. (I await correction from any real chemists who might happen by, though.)
I’m spending too much time thinking about this, but I found this “ghetto” flash chromatographymethod… it could work. A quantitative analysis would still have to be made to determine the amount of vanillin in the initial solution, but it should be pretty straightforward if you have access to something to measure it with.
A TLC plate could tell you how pure each fraction was.
The combo of flash+TLC chromatography is how I isolated the compounds I was working with in my undergraduate organic chemistry lab. We used a rotary evaporator to remove solvent. In this case, we did the identifications by NMR, but we were looking for a particular isomer.
Man, I can’t wait to have kids and try out kitchen-chemistry 
On thinking about it, the answer is simple - use the camera to measure the colour of all of the samples simultaneously (use an eyedropper tool in a photo editing program to sample the colour of a few pixels from each one)