What is the substance that tumors release that stimulates growth of blood vessels but suppresses its release from other tumors?
One very important signal for angiogenesis are the Vascular endothelial growth factors (VEGF)  (a small group of ~5 similar proteins). They generally promote generation of new blood vessels &amp; migration of these towards the source of VEGF expression.
Blocking VEGF signalling (mostly be blocking the receptors with antibodies) has been a pretty good strategy in cancer treatment, since tumors can't grow over a certain size without blood supply by new grown vessels. 

However, I am not aware of any blood-vessel specific signalling mechanism that stops it's own expression at a secondary site. (This may be possible though, especially for specific signals)