Voltage expression in a circuit
As the voltage V1 does not have a common node with Vs, you can not simply use one voltage divider formula, you actually have 2 voltage dividers. 
By redrawing the circuit that becomes more clear:







simulate this circuit &ndash; Schematic created using CircuitLab

So what you need to do is calculate the voltage of both voltage dividers. The voltage difference between the two dividers is V1.