When excavating house foundations how much more would it cost to use hydro-vacuum excavation rather than traditional hydraulic digging?

If the question was "can a hydrovac do it", the answer is "Yes". However, since you wish to compare alternatives based on cost, I offer the following comments.

Short answer: Unless there are challenging circumstances (limited access to proposed dig site, existing buried utilities, landscaped property), hydrovac would likely cost more than traditional mechanical excavation methods.

Longer answer: The cost to excavate a house foundation by hydrovac will depend on:

1) Soil conditions

2) Volume to be excavated

3) Rate of production per load (for the particular hydrovac used)

4) Time per load

5) Time to dispose of material

6) Cost of disposal per load

7) Hourly rental rate of the hydrovac

There are many hydrovacs available in the market, however not all hydrovacs deliver the same rate of production (nor do they cost the same).

Also, you could save on restoration costs (compared to traditional means) if the work were to be done at an already developed property. Hydrovacs can work at distances greater than 450' from an excavation site, thereby minimizing the footprint left on existing property.