Postdoc in the US : what salary should I ask for
First, it is important to understand that a visiting assistant professor position isn’t a postdoc in the sense that you are being hired primarily for research and based on your research background.  This position will be funded by the institution rather than by grants. Teaching will be an important part of the job, and your teaching experience will be important in the selection process.  

The American Mathematical Society publishes an annual survey of faculty salaries.  I’d encourage you to look at the survey to get a sense of the range of faculty salaries in the US. Figure that a VAP will be paid slightly less than a first year tenure track assistant professor.  If I had to guess, I’d expect a position like this at UD to pay somewhere between $60K and $70K.