Do pharmaceutical companies have an obligations to develop and produce vaccines for diseases (such as malaria and tuberculosis) that exist primarily in poorer countries?
No, pharmaceutical companies do not have an obligation to do this any more than oil companies have an obligation to give fuel to people in poorer countries or farmers in rich countries have obligations to grow food and give it to people in those countries. There is no obligation for a firm to use its resources to create something that will be of no economic benefit to it.
It would certainly be good if pharmaceutical companies were to develop these vaccines. However, they are under no obligation to do so. What should happen is that governments, including both the governments of the poor countries and those of rich countries, should put up money to help defray the costs of developing and producing the vaccines. Other organizations like the UN and private charities should help. The companies should, as a charitable gesture, provide the vaccines at low cost. However, this is a very different thing than saying that the firms have an obligation to, at their own expense, create vaccines for people in poorer countries.
Pharmaceutical companies have NO obligation to find vaccines or treatments for any disease, but they do have motive to do so. The purpose behind any business is to use one's resources to fulfill someone else's need, and thus earn a profit. Pharmaceutical companies are the same way. They have the resources to develop medicines to treat a variety of diseases, and selling them at affordable prices in areas where they are needed earns them a profit.
However, not all diseases are the same. Doctors would love to find a vaccine against HIV, but they've been unable to find an effective vaccine because of the way the virus works. The common cold has no vaccine because there are so many strains and they change so rapidly that the vaccine becomes ineffective within weeks. Malaria and TB have similar problems, effective vaccines elude them, and there are only so many known treatments.
Do some research on the details, and opinions are your own.