Caramelization occurs at the melting point of sugar. When a sugar molecule hits the appropriate temperature, it melts. This is similar to ice turning to water above 32 F (0 C). It will take some time for all of a given amount of sugar to melt, but this is relatively insignificant compared to ice melting due primarily to the vast amount of heat involved to melt sugar. Once melted there is no need to maintain it at a specific temperature.
However, time is a factor in achieving the desired temperature for your melted sugar.
For sucrose:
- At 356 F (180 C) you have light caramel - it's a pale amber to golden brown in hue
- From 356 F to 370 F (180 C to 188 C) you have medium caramel - this is a golden brown to chestnut brown hue.
- From 370 F to 400 F (188 C to 204 C) you have dark caramel - this is very dark, bitter, and smells a little burned - this is used for coloring only
- At 410 F (210 C) you have monkey's blood - it tastes like burning and the sugar breaks down to carbon
Another thing to be aware of is that caramelization is often mistakenly attributed to the browning of meats, nuts, or bread crust. This is actually an entirely different process called the maillard reaction which requires specific enzymes to be present, and occurs at different temperatures than caramelization.
I am not sure what your specific problem is, so here is some general advice on making candy and caramel.
As for control, the normal method is to control it with a thermometer. (They are even called "candy thermometers"). The thermometer should have a very fast reaction time, and be capable of measuring in fractions of grades (one digit after the decimal point is enough even for Celsius measurements).
You should also have a source of heat which is as responsive as possible. Induction and gas work best. If using resistive electricity, be prepared to yank the pan away from the heat a degree or two before the temperature limit is reached (you must calculate some carryover: The heat from the pan will continue to heat up the syrup even after the source of heat has been removed. Expect more carryover from copper core than pure steel, and more from steel than from aluminum).
The caramelization of pure sucrose begins at around 160°C, but if you mixed in other types of sugar, it is different. For any type of suger, with rising time and rising temperature, you get a darker, less sweet caramel. It is up to you which kind you want, but the very light kinds don't have the complex aroma of the middle hued ones, and the very dark ones are somewhat harsh.
There are charts for the stages of sugar syrup (threads, soft ball, etc) and the corresponding temperatures. They determine not only the degree of caramelization, but also the hardness of the finished candy. It is easy to find an English language one with Google, try this one. A bit of search should turn up some which contain photographies of the candy instead of penciled illustrations. Use such a chart when making candy, you need it.
Another variable in candy comes with your ingredients. Acids will soften caramel. Enough of them will keep it liquid. Milk and cream will soften it too. Pay attention to when and how you are adding them. If you let dairy products sit in the pan for too long, they'll develop their own scalded milk flavor. Also, don't add cold liquids to hot caramel, it will seize up.
That's about it, if you can point out specific problems you are having, it will be easier to help you.
Best Answer
Most of the aromatic compounds contained in vanilla are highly volatile and / or degrade in high heat, so I would add the vanilla after caramelization.