Ok, I'm drawing a blank on how to handle this... I added a table of zip codes, which also have fields for the longitude and latitude. That's rougly 80,000 records. I have the calculation to find the distance between two zip codes (based on longitude and latitude).

Now, the hard part:

I have a Jobs table, which has a zip code field, and a Vendors table which has a zip code field. When I'm in the Jobs layout, I want to have a portal that shows all my vendors in a 100 mile radius. Any ideas?

Look into using a geo box instead of a circle, it's much less computationally intensive and easier to implement.

To implement a geobox, take the centre point client lat lon and, based on the "radius" calculate the corresponding Lats and Lons for the four corners. Use these as the bounding values for a multi-predicate relationship to the vendors.

The GeoBox custom function on Brian Dunnings site might make implementation easier.