Here's a thought straight off the top of my head...

The chances are your sensors will produce an output proportional to Vdd applied. The ADC's will give you a result proportional to Vdd. You therefore probably don't have to worry about a too stable Vdd as the two (ADC and sensor) will still be in proportion respective to Vdd. Anyway, since when did regulators ever give you 5.000v? So run your product at 3v or 4.5v and lose the hassle, expense and power consumption of the whole Regulator circuitry.