Current plants already react to light; every plant will "seek out" light by growing their shoots towards a light souce, and grow roots in the other direction. You can see it yourself if you grow plants indoors, how they will apear to lean towards windows as they grow. Plants can react even faster, with sunflowers slowly rotating as the sun progresses across the sky, or flower blooms opening during the day and closing at night. Carolus Linnaeus proposed a Horologium Florae made of beds of fourty-three types of flowers that would open and close at different times of day, like a sundial of blooms.
The mechanisms are there, corresponding to the light-sensitive patches of skin on animals. You could use mad science to make these processes more efficient (along with the rest of the plant metabolism so it can react at animal-like speeds), or a lot of time and environmental handwavium for encouraging light-sensing traits in plants. Animals improved their light-sensing by having a polyp half-covered in sensors and a small aperture, using part of their meat as a pinhole-camera; it's very efficient for not much expense so I'd expect the same mechanism in other creatures that will have distance vision. A plant with a pinhole-camera pod would have a much better time of it with a focusing lens, as it would resolve fine detail faster and with a smaller pinhole-pod, and the lens can physically close the pinhole keeping the pinhole-pod clean of dust or other opaque substances that would cover the light-sensitive parts. Perhaps the first lenses would be a drop of water, as they're very transparent and flexible, later evolving to a transparent sap that will hold its shape and less likely to evaporate.
Now, where's the organ that collects this sensory data and turns it into perception? and does it quickly? That's the real thistle of the puzzle.