Try countering with:
1) Brady/Harvin for Vick/Megatron, OR
2) Brady/Marshall for Vick/Cruz.
I would start with the first offer. Anyway you can get trade value with Vick and get Brady would be good for your team. Marshall was hyped on paper, but seems to be limited in fact by Cutler. The trade AS IS, isn't too bad, but I would try a counter offer and see what you get.