Can we use cancer cells to generate blood vessels?
Using cancer cells in this manner would be far too dangerous, as there is nothing to prevent the cells from implanting and forming new tumors. Fortunately, we already know a great deal about angiogenesis, which is the process by which new blood vessels are formed or grown out of existing ones. If you want more details on how the process works, the article I linked to goes into a good amount of detail regarding the key players and how they interact.

It would be much easier (and safer) to use our knowledge of the cytokines and growth factors responsible for blood vessel formation to develop biological therapies that could potentially be used to treat varicose veins. However, current therapies can be quite successful, and there may not be a large enough market to justify the quite large expense required to develop angiogenesis-based therapies.

Unless a patient has some sort of genetic or other issue, angiogenesis is already involved in wound healing. As new tissue grows back, capillaries and perhaps slightly larger blood vessels expand out from the nearest sufficient vein/artery pair to vascularize the new cells. There may be issues with patients who are taking angiogenesis inhibitors as part of a therapy plan for cancer, for example, in which case compromises would have to be made if necessary.