Currently PI Vision uses PI Data Services an application which is was built for data access for a number of web services and relational databases. In general, I would highly recommend keeping with the PI Web API which is actively being developed. We're in the process ourselves to move to the data access to the PI Web API for PI Vision as well.
That being said, while I wouldn't recommend switching to using the same data retrieval service as PI Vision perhaps we can get a better idea of what's going on with the call you're making to the PI Web API. Would you be willing to share the details of the query you're making to the PI Web API? Could you also share the end goal you're looking to accomplish with this (i.e. interpolate values for this formula attribute every hour on the hour). I tried to reproduce this on my system just now and I wasn't able to.