Just to review, you digested your vector with SmaI, and ran that product against a non-digested vector sample? I would guess that your SmaI digestion linearized your vector, while your vector stock is, at the very least, 'relaxed' or 'nicked' due to nuclease contamination/ degradation and that could explain the difference in migration.

Relaxed/ Nicked circular DNA tends to migrate higher/ slower than linearized DNA. Supercoiled DNA will migrate lower/ faster than linearized DNA.

Here's a great overview of the different possible conformations of plasmid DNA: http://www.escience.ws/b572/L2/L2.htm

Could you verify the ladder composition? Also, it looks like you may have some linearized (denatured) product (if, in fact, your SmaI digestion went to completion) in your stock/ undigested sample, as there is a faint band migrating at the same distance on the gel as your SmaI digested sample. Nuclease contamination or an extended alkaline lysis during plasmid extraction and purification can cause denaturation.

Your samples are definitely overloaded, though. Try running 1/10 of what you originally put on the gel. This may help overall resolution. Good luck!