cirrus/wolfson wm8731 - Connecting Analog Input to Codec
No, you do not need to supply a voltage to VMID; the voltage is supplied by an internal voltage divider which drives VMID to a voltage midway between AVDD and AGND. This internal circuitry is shown in the upper-left corner of the block diagram on page 1. (This kind of block diagram is only an approximation or a summary of the real internal circuitry, but gives a good overview.) The electrical characteristics table on page 11 "Analogue Reference Levels" lists VMID pin voltage VVMID as typically AVDD/2 and the resistance of the divider RVMID as typically 50Kohm. (Resistors inside an IC are implemented with thin or thick films, with nearly constant ohms per square within the IC. So as long as both resistors are the same width and length, they will have equal value. So the AVDD/2 midpoint will most likely be very accurate even if the RVMID resistance has some variation.)

As mentioned in the question, external bypass capacitors between the VMID pin and the lower supply rail provide a lower-impedance path for higher-frequency noise. These are clearly indicated on page 60 under "Applications Information / Recommended External Components". The 0.1uF capacitor should be ceramic (for low ESR) and must be located within about 5mm of the WM8731 -- if it's too far away, it will not be effective because of too much PCB route inductance. For the same reason, it's best to avoid routing high-frequency bypass capacitor routes though vias, because PCB vias are inductive. 

Also note that the schematic on page 60 shows the ADC analog inputs as well as the DAC analog outputs are AC-coupled through a series-connected 1uF capacitor. So there is no need to add offset to the inputs, the DC offset is removed. This codec is meant for audio applications, it's not intended to be used for setting or measuring DC levels. The electrical characteristics table specifies dynamic parameters (SNR, THD) but does not specify any static parameters (INL, DNL, offset voltage, gain error).