Will a vanishing twin (lost at 7 weeks) affect the results of first trimester Down's syndrome blood testing?

Unlikely. These tests rely on the active production of various markers in the blood and so a demised twin this early on will be unlikely to affect the test. These tests however, should be interpreted in the setting of a screening ultrasound to help better understand the risk. That being said, no test is ever 100% accurate, though they are useful. Consultation with a genetic counselor will help.