Poor girl.
I'm not an expert, but bumble foot tends to be more obvious. However, a poorly healed break in the leg or toe can lead to discomfort walking or simply physical trouble walking (without pain). It could be a joint further up causing the issue. Is the leg swollen, warm to the touch, or shaped differently than the leg she is not limping on?
As far as preventative care, good food, clean water, cleaning living conditions, fresh air (good ventilation) and as little stress as possible leads to happy, healthy chickens. Any vaccinations are generally given once only to chickens and any other medications are only necessary for chickens that are sick.
If you are worried about worms you can look into deworming, but regular cleaning of the coop prevents reinfection, which is what leads to an overload of parasites.
Your chickens are lucky to have someone who cares. : )
Best of luck!