Astra uses only 2 wires if remember correctly so wont be different in vectra dig out older posts and you will find how to wire it
Most important thing is to use good shielded cable
Had problems when was fitting mine first no mic at all then it was not too quiet, resoldered mic pins, new shielded cable around 3m but used only about 2m left with good spare, crimped on terminals and soldered them all cabling was fitted like factory from radio back to connector in roof than mic in connector
And woila it worked nice and loud