You have some options:
1. Use +5V Vdd as Vref and use it as is with reduced sensitivity
2. Use a lower external VRef (which could be as simple as a voltage divider from Vdd)
3. Use Vdd as Vref and an opamp setup for DC gain to amplify the signal

However, all this is the easy part. When measuring AC voltage/current you will need to sample both many times per half-cycle (what frequencies are you dealing with?) and average the readings over each half-cycle. This also means you'll need to detect zero crossing.