two good ways to go to Vallecas:
1. catch the bus 154 (MIGHT still be 154, just ask...) in front of the museo reina sofia at the bus pool (or @ monlcoa station), and take the bus to vallecas this way, or,
2. take the metro to puente de vallecas and ask for your street then.
hope this helps... we'll see if madridman comes back with any other info... (i haven't been to vallecas in a couple of years...)